According to SDA understanding did Methuselah preach just before the flood?
The short answer is yes.
God Preached Through Methuselah, Noah, and Others—God granted them one hundred and twenty years of probation, and during that time preached to them through Methuselah, Noah, and many others of His servants. Had they listened to the testimony of these faithful witnesses, had they repented and returned to their loyalty, God would not have destroyed them (The Review and Herald, April 23, 1901). - SDA Bible Commentary, vol. 1 (EGW)
How can she assert this is not known. It would seem that Methusaleh possibly died in the same year as the flood and possibly may have perished in the flood, in which case one would wonder that if he 'preached alongside Noah' then why was he permitted to perish in the flood? There is no indication in Sacred Scripture that he did preach, so any supposition comes from outside of the bible.
Thus she would have to base this on extra biblical sources.
Even her supportive quote from 1 Peter 3:18-21 does not fulfill that evidence and thus would simply be her own personal take on the subject.
18 For Christ also suffered once for sins, the righteous for the unrighteous, to bring you to God. He was put to death in the body but made alive in the Spirit. 19 After being made alive,[a] he went and made proclamation to the imprisoned spirits— 20 to those who were disobedient long ago when God waited patiently in the days of Noah while the ark was being built. In it only a few people, eight in all, were saved through water, 21 and this water symbolizes baptism that now saves you also—not the removal of dirt from the body but the pledge of a clear conscience toward God. It saves you by the resurrection of Jesus Christ.
Even Catholicism does not acknowledge this as there is so little known about Methuselah, biblically speaking.
One of the Hebrew patriarchs, mentioned in Genesis 5. The word is variously given as Mathusale (1 Chronicles 1:3; Luke 3:37) and Mathusala. Etymologists differ with regard to the signification of the name. Holzinger gives "man of the javelin" as the more likely meaning; Hommel and many with him think that it means "man of Selah", Selah being derived from a Babylonian word, given as a title to the god, Sin; While Professor Sayee attributes the name to a Babylonian word which is not understood. The author of Genesis traces the patriarch's descent through his father Henoch to Seth, a son of Adam and Eve. At the time of his son's birth Henoch was sixty-five years of age. When Methuselah had reached the great age of one hundred and eighty-seven years he became the father of Lamech. Following this he lived the remarkable term of seven hundred and eighty-two years, which makes his age at his death nine hundred and sixty-nine years. It follows thus that his death occurred in the year of the Deluge. There is no record of any other human being having lived as long as this for which reason the name, Methuselah, has become a synonym for longevity.
The tendency of rationalists and advanced critics of different creeds leads them to deny outright the extraordinary details of the ages of patriarchs. Catholic commentators, however, find no difficulty in accepting the words of the Genesis. Certain exegetes solve the difficulty to their own satisfaction by declaring that the year meant by the sacred writer is not the equivalent of our year. In the Samaritan text Methuselah was sixty-seven at Lamech's birth, and 720 at his death.
Ellen Gould White could have based this assumption as coming from a number of sources, but not from the bible as the majority of Christians hold it to be.
Methuselah is a biblical patriarch mentioned in Genesis 5:21–27, as part of the genealogy linking Adam to Noah. The following is taken from the New Revised Standard Version of the Bible:
21 When Enoch had lived sixty-five years, he became the father of Methuselah.
22 Enoch walked with God after the birth of Methuselah three hundred years, and had other sons and daughters.
23 Thus all the days of Enoch were three hundred sixty-five years.
24 Enoch walked with God; then he was no more, because God took him.
25 When Methuselah had lived one hundred eighty-seven years, he became the father of Lamech.
26 Methuselah lived after the birth of Lamech seven hundred eighty-two years, and had other sons and daughters.
27 Thus all the days of Methuselah were nine hundred sixty-nine years; and he died. - Genesis 5:21–27
In the Bible
According to the chronology of the Bible, Methuselah died one week before the Great flood; He was also the oldest of all the figures mentioned in the Bible. Methuselah is mentioned once in the Hebrew Bible outside of Genesis; in 1 Chronicles 1:3, he is mentioned in a genealogy of Saul. Methuselah is mentioned a single time in the New Testament, when the Gospel of Luke traces Jesus' lineage back to Adam in Luke 3 (Luke 3:23–38)
In other religious texts
The apocryphal Book of Enoch claims to be revelations of Enoch, transcribed by him and entrusted to be preserved for future generations by his son, Methuselah. In this book, Enoch recounts two visions he has had to Methuselah. The first is about the Genesis flood narrative, and the second chronicles the history of the world from Adam to the Last Judgment. In the latter vision, men are represented as animals – the righteous are white cattle and sheep, the sinners and enemies of Israel are black cattle and wild animals. Following his father's death in the Book of Enoch, Methuselah is designated by God as a priest, while Methuselah's grandson, Noah's brother Nir, is designated by God as his successor. In Slavonic Enoch, Methuselah asks his father for a blessing, and is given instructions on how to live righteously. After their father ascends into heaven, Methuselah and his brothers build an altar and made "a great festivity, praising God who had given such a sign by means of Enoch, who had found favor with Him."
The Book of Jubilees presents itself as "the history of the division of the days of the Law, of the events of the years, the year-weeks, and the jubilees of the world" and claims to be a revelation of God to Moses, given through the Angel of the Presence in addition to the written Law received by Moses on Mount Sinai; and, while the written Law was to be imparted to all, this was to be a secret tradition entrusted only to the saints of each generation, to Enoch, Methuselah, Noah, and Shem, then to Abraham, Isaac, Jacob, and Levi, and finally to the priests and scribes of the latter times.
Rabbinic literature states that when Noah was 480 years old all the righteous men were dead, except Methuselah and himself, who were of immense stature as they were descended from the sons of God. At God's command they both announced that 120 years would be given to men for repentance; if in that time they had not mended their evil ways, the earth would be destroyed. But their plea was in vain; even while Noah was engaged in building the ark the wicked made sport of him and his work, saying: "If the Flood should come, it could not harm us. We are too tall; and, moreover, we could close up with our feet [which were of monstrous size] the springs from below." They resorted to these tactics; but God heated the water, and their feet and the flesh of their bodies was scalded.
The 17th century midrashic Sefer haYashar ("Book of Jasher") describes Methuselah with his grandson Noah attempting to persuade the people of the earth to return to godliness. All other very long-lived people died, and Methuselah was the only one of this class left. God planned to bring the flood after all the men who walked in the ways of the Lord had died (besides Noah and his family). Methuselah lived until the ark was built, but died before the flood, since God had promised he would not be killed with the unrighteous. The Sefer haYashar gives Methuselah's age at death as 960.
Methuselah (Arabic: Mattūshalakh) is also mentioned in Islam in the various collections of tales of the pre-Islamic prophets, which also say he was an ancestor of Noah. Furthermore, early Islamic historians like Ibn Ishaq and Ibn Hisham always included his name in the genealogy of Muhammad.
The Book of Moses, a Mormon text, says that after Enoch and the City of Zion were taken up to heaven, Methusaleh stayed behind; this was so that God's promises to Enoch – that he would always have descendants on earth and that he would be an ancestor of Noah – would be fulfilled. The Church of Jesus Christ of Latter-day Saints further teaches that Methuselah was a prophet.