First of all, Jesus addressing the Father as "You, the only true God" does not semantically mean "that no one else is true God except the Father." That could be inferred only if Jesus had said "You, Who alone are the only true God". It is most evident that the statement "the Father is the only true God" does not imply the statement "the Father alone is the only true God".
Addressing the question, the meaning of Jesus calling the Father "the only true God" is just to confirm that there is only one omnipotent eternal God, Creator, Sustainer and Lord of everything that exists outside Him, which is a truth accessible by the natural light of human reason (Rom 1:19-21) and explicitely revealed in the OT:
"You are my witnesses," declares the LORD, "and my servant whom I have
chosen, so that you may know and believe Me and understand that I am
He. Before Me there was no God [el] formed, neither shall there be after Me. (Is 43:10)
"Thus says YHWH, the King of Israel and his Redeemer, YHWH of hosts:
'I am the first and I am the last, and there is no God besides Me. (Is
Do not tremble and do not be afraid; have I not told you from of old
and declared it? And you are my witnesses! Is there a God [eloah] besides Me?
There is no Rock; I know not any.'" (Is 44:8)
"I am YHWH, and there is no other; besides Me there is no God. I will
gird you, though you have not known Me; (Is 45:5)
"Declare and set forth your case; indeed, let them consult together.
Who has announced this from of old? Who has long since declared it? Is
it not I, YHWH? And there is no other God besides Me, a righteous God [el]
and a savior; there is none except Me. (Is 45:21)
Now, it is well-known that the name Elohim, literally "the gods", has two meanings in the OT: a) the only omnipotent, eternal Creator God, YHWH, in which case the plural has a majestatic sense and the name is the subject of a singular verb, and b) the gods, either existing super-human entities created by and subordinated to YHWH (in pre-exilic texts only, of which I make the case below) or the imaginary gods of the gentiles, in which case is the subject of a plural verb.
Thus, verses like Is 44:6 and 45:5 which say literally "besides Me no Elohim/elohim" can be understood in either of two senses, depending on the sense of Elohim/elohim: with "Elohim" in sense a, as "besides me no omnipotent, eternal Creator God", and with "elohim" in sense b, as "besides me no gods", so that someone intent on affirming that post-exilic Jews did NOT reserve the term "god" for YWHW can reject these verses as proof to the contrary by saying that they use Elohim in sense a only, and therefore do not preclude the existence of lesser, subordinated, created gods which should not be worshipped.
Therefore I emphasized above the instances where "God" translated a singular Hebrew name, either "el" (Is 43:10) or "eloah" (Is 44:8), which I gave between . From these instances, it is clear that for post-exilic Jews all names referring to a divinity, both the majestatic plural "Elohim", which in Greek would be "ho Theos", and the singular "El" and "Eloah", which in Greek would be "Theos", were reserved to YHWH. The other existing superhuman real entities, the angels, were created by YHWH and wholly subordinated to Him, so that they could not be called "gods". And the "gods" of the idolatrous peoples were not real: "For all the gods [elohe] of the peoples are idols, but YHWH made the heavens" (Ps 96:5).
I wrote the 3 previous paragraphs to preemptively dispel the notion that in Jn 1:1 the final unarthrous "Theos" is actually "theos" and refers to a created superhuman entity, as non-trinitarians (or more precisely, as people denying the numerical identity of ousia between the Father and the Son) posit. But Jesus calling the Father "the only true God" in Jn 17:3 achieves exactly the same: because if the Father is "the only true God", then there are only two possible ways to understand the final clause "the Word was God" (or "the Word was god") in Jn 1:1:
A. The homoousian way: the Word is all that God the Father is (except Father), i.e. the Word is also the only true God;
B. The heteroousian way: the Word is a fake god.
So, while the meaning of Jesus calling the Father "the only true God" is quite straightforward and not new to the NT, the reason why John recorded it is to preemptively dispel any heteroousian reading of the passages where Jesus is called theos. Because if there is only one true God, the Father (also in 1 Cor 8:6), and the Son is not all that the Father is (except Father), then the Son is a fake God.
Since I have used the noun ousia by itself and composed with homo/hetero, I note that it is the noun directly deriving from the verb "to be" in Greek, first person singular "eimi" and present participle "on", meaning what a subject is, and that Jesus used that verb to state his divinity in John's Gospel in two ways: 4 times with Himself as subject without further qualifications, as "I Am", "Ego Eimi", clearly in the sense of the Name of God in the first person revealed in Ex 3:14, in Jn 8:24,28,58 and Jn 13:19, and once with Himself and the Father as subject, "I and the Father are one" in Jn 10:30. Refraining from using a noun which derives directly and naturally from a key verb used in the Gospel is unnatural and unwarranted.
To place the above in the context of other NT passages containing "Theos", we must first note that the NT uses two terms, "ho Theos" and unarthrous "Theos".
The term "ho Theos" or its genitive "tou Theou", dative "to Theo", or accusative "ton Theon", refer to God the Father, except in the 5 passages where it refers to the Son, none of which calls Jesus simply "ho Theos" without qualification: Mt 1:23, Jn 20:28, Ti 2:13, 2 Pe 1:1, 1 Jn 5:20. But even in these cases "ho Theos" refers always to a divine Person, not to the divine ousia.
On the other hand, unarthrous "Theos" can refer to either
the one and only divine ousia, what each divine Person Is, in which case it is the attribute of a copulative sentence whose subject is the Son (Jn 1:1, Rom 9:5) or the subject of a passive predicative sentence, or
a divine Person, usually God the Father when it appears without qualification or the Son in "monogenēs Theos" (Jn 1:18).
So, Jn 1:1 says:
"In the beginning was the Word, and the Word was with God the Father, and the Word was all that God the Father was (except Father)."
Where from monotheism, "all that God the Father was" is understood in a sense of numerical identity, not of merely qualitative identity. "I and the Father are one" (Jn 10:30), not "I and the Father are equal". "Homoousios", i.e. "of the same ousia" (numerical identity), not "isoousios", i.e. "of identical ousiai" (merely qualitative identity).
In terms of the four possible theological positions: consubstantial Trinity (nicene orthodoxy), tritheism, Arianism and modalism:
Thus, you can either hold the consubstantial Trinity, whereby the Son is also the only true God, the Father's design "that all may honor the Son, even as they honor the Father" (Jn 5:23) implies worshipping the only true God, and the statement "I and the Father are one" is meant in an ontic sense,
or hold Arianism, whereby the Son is a fake god, the Father's design "that all may honor the Son, even as they honor the Father" (Jn 5:23) implies divinely mandated idolatry, and the statement "I and the Father are one" is meant in a merely moral sense.