According to the Temporal Integrity Commision on the 29t century, Star Trek: First Contact
is actually a "pogo paradox"...
DUCANE: Let's see how much you've assimilated. The Dali paradox.
SEVEN: Also known as the Melting Clock Effect. It refers to a temporal fissure which slows the passage of time to a gradual halt.
DUCANE: The Pogo Paradox.
SEVEN: A causality loop in which interference to prevent an event actually triggers the same event.
DUCANE: Excellent. Can you give me an example?
SEVEN: The Borg once travelled back in time to stop Zefram Cochrane from breaking the warp barrier. They succeeded, but that in turn led the Starship Enterprise to intervene. They assisted Cochrane with the flight the Borg was trying to prevent. Causal loop complete.
DUCANE: So, in a way, the Federation owes its existence to the Borg.
So in-universe, First Contact
and by extension Enterprise
always happened the way we saw.