It means it's in 'the not too distant future' of the flashback we just saw. Any other way is just nonsensical.
The 'not too distant' future of the early 1940's would be the late 40's, the 50's or 60's at the most. It is
utterly nonsensical to suggest that the intention of X1's caption, the
proof that you asked for, was anything other than to state the events about to unfold are in the not too distant future of our own 'present day'.
Seriously, they'd have just said 50 years later if your interpretation had any merit all.
They don't have it in X2. Does that make 2003 their present day?
Having established it was all not too distant future in X1 (a vague description as I'll elaborate on later) it wasn't needed. The story for X2 picked up in that films present day, rather than after a flashback as both X1 & X3 did.
Does that mean that 'the not too distant future' means '3 years after this date' But wait. Then they have it in X3. Does that mean that X3 takes place in 2009? Does that make it 6 years after X2? Why do Bobby, Kitty, Rogue, Peter, and several other characters from X2 only look a few years older and why are some still even in school at all?
Ok. 'Not too distant future' is a
deliberately vague description that can be a no more than a few years or extend to a few decades. It's a
commonly employed narrative term used in Sci-Fi so a story set in the future can stand alone and not get dated too quickly. Verhoven's Robocop is an example of using this. Blade Runner is an example of
not using it (they gave that film a far too near sighted 2019 setting). This extends to sequels as it allows the narrative more freedom and doesn't tie the future stories down to any dates mentioned previously that might then need explanation down the line (Like the Terminator films, which had to shunt 'Judgement Day' down the timeline with each film after T2).
Bottom line: in theory X1, X2 and X3 can
all be said to
still be in our near future as they gave no specific year for
any of them.
I suspect though that with DOFP the ages of Stewart & Mckellan, and their respective characters, won't give them much more wriggle room for placing anything too much further down the line.