The Preterist interpretation of the book of Revelation basically claims that almost everything in Revelation was fulfilled in past history during the first century of the church. With regard to Luke 21 Jesus was foretelling the destruction of the city of Jerusalem and the awful tribulation that Jews would suffer as it happened. But Jesus told his followers about fleeing so as to avoid it, and when they understood how his instructions could be carried out when Roman troops suddenly lifted their siege, they fled. Later, the city was besieged again (in A.D.70) and that’s when terrible tribulation was the portion of all the Jews still in the city. Obedient Christians were spared that by obeying Jesus.
It would appear that in order to fit Luke 21’s mention of tribulation into the Preterist scheme, they think the Book of the Revelation was written before A.D.70. I am not a Preterist myself, but assume that to be the case, given what these Preterists say about Revelation 7:9-17 (the great crowd who came out of the great tribulation) and its application. In “Revelation – Four Views” the column objectively stating the Preterist case says:
Having shown John the Jewish saints who would escape the Tribulation
of A.D. 70, the Lord now shows him the great throng of Gentiles who
will be saved as a result of God’s disowning His rebellious wife and
children and seeking a new family… These ones come out (literally,
“are coming out”) of the great tribulation (v.14) in the sense that
their inclusion in God’s kingdom resulted from that event, at which
time Judaism came to a formal end and the universal gospel was
proclaimed to all nations.
Alternatively, they are Christian martyrs, slain by certain Roman
emperors after the fall of Jerusalem (the view of Adams, David S.
Clark, and others). Adams writes that:
“the vast Gentile multitude, coming out of the portending Roman
persecution, is also introduced at this point… Note the heavy emphasis
upon the glorious gains and blessings of faithful martyrdom in verses
9, 15, 16, and 19… It has been thought by some that the terms used to
describe this multitude are too inclusive or universal to fit the
historical view of these chapters. In answer it may be said that the
terms used to describe the multitudes that were in Jerusalem at
Pentecost were almost as universal; for it is said that there were
Jews from every nation under heaven.” Edited by Steve Gregg, pp 134 &
136, Thomas Nelson Publishers, 1997
The Preterist approach to chapters 4 to 7 in the Revelation is, according to Gregg’s summary:
“The unsealing of the scroll represents the judgment of God upon
Jerusalem (A.D. 66-70); 144,000 Judean Christians escape to Pella. The
four horsemen represent the Roman invasion of Israel to quell the
Jewish rebellion (A.D.66), bringing bloodshed, civil war, famine,
death, and ultimately the fall of Jerusalem in A.D.70.” (Ibid. p83)
This means that the Preterist view of Revelation 7:14 (the only time ‘tribulation’ is mentioned) does not look forward to a still-future time before Jesus suddenly returns. They would read Luke 21:36 and its mention of tribulation as Jesus foretelling the troubles leading up to A.D.70. So, when Jesus says to Christians back then to watch and pray that they will escape what is to happen, it is a warning not to slip into the ways of non-Christians detailed in verse 34. Those who remain alert will escape that tribulation.
However, many Christians today who do not have that Preterist view see a two-fold application of Jesus’ prophecy in Luke 21, interpreting it quite differently as having an initial fulfilment in A.D. 70 and a still-future application in the time leading up to Christ’s second appearing. You would need to ask a different question to deal with that, citing particular verses in Revelation, because the explanation I have quoted (from Preterists) sees no problem as it all happened way back then, and Christians who remained alert and obeyed Christ’s instructions did, indeed, escape what happened in A.D.70 (v.36).
I am not arguing the case for Preterists, simply using quotes from a book giving something of their view in order to answer your question. I think that Revelation clearly shows that many Christians "come out of the great tribulation" (7:14) which means that they went through it.
EDIT after seeing the Q was changed from "For Preterists..." to "For Pretrib believers..." Anyone interested in comparing the Preterist view on such texts with Pre-tribulation views could check this link. It is written by a man who used to write Pre-trib articles but then changed to accepting the Preterist view.