The poster ist a preterist?
Why do you figure it was a preterist? (Many Catholics are partial preterists: they believe that there is a sense in which a prophesy is describing current conditions... but usually to demonstrate a larger theme which will come to full fruition at a future date. For instance: the Beast of Revelations is Nero, whose Hebrew name (QSR NRN) adds up to 666. BUT Nero serves merely as a fore-runner for an apocalyptic repression.)
I’m not sure how preterism would affect the dating of the Gospel of John. I can agree that a date that early seems a little odd: the closing of the gospel addresses an expectation that the end of the world would occur before the death of Peter. That would seem to suggest that it was written after Peter died.
What I have a problem with is that both sets of dates are presented as if all the books were created instantaneous. If the date ranges are meant to indicate a process, they look more like a range of estimates. Matthew, John, and Revelations were created by a process which probably took decades:
John, for instance, is probably a compilation of liturgical readings created by John. Evidence for the gospel being a compilation includes its disjointedness: its individual episodes are very highly formed, and appear thematically arranged, but collectively overlap each other’s time frames. For instance, Jesus concludes his last supper discourse sermon in Ch. 14 to depart presumably for the Mount of Olives, but then in chapter 15 he continues in what appears to be a last supper discourse.
Revelations also seems to have a similar disjointed timeline, in spite of very heavily thematically unified readings. Some have even suggested that Revelations seems to include the writings of a previous author (John the Baptist?). (This shouldn’t be shocking: IIRC, John 1:1-5, 10-12, 14 a pre-Christian prophesy alluded to in the Dead Sea Scrolls, which is fulfilled by Christ, as explained in John 1:6-9, 13, 15-18.)