Well, clearly the 20th Century we lived in and the one that is in Star Trek's past are not the same, and never really have been.
The first obvious difference being that in our timeline Star Trek is just a TV show, while it is the reality happening in the 23rd century in their timeline. This is why Kirk and Co. do not get mistaken for Shatner and Co. in STIV and mobbed by obsessive trekkies.
The Eugenics War of the 1990's did not hapen in our 20th century like it did in theirs, nor do we have any spacecraft as advanced as the one Khan escaped with in their 1990's.
was meant to be our future. Which is why, when its speculation about what the future will be is shown to be wrong, it should be ignored.
A foolish consistency ( or continuity
) is the hobgoblin of little minds.