There's far too much mental gymnastics involved in explaining all the continuity errors in Enterprise. In reality it's down to poor writing but in universe the best and most reasonable explanation to me is that it takes place in a different universe. Some people complain that the problems are no worse than the stuff in TOS but at the time TOS wasn't working with an established universe, Enterprise didn't have this excuse.
TOS has a series bible, detailing the workings of the Enterprise and it's universe (a version of which can be found in The Making of Star Trek
), so they've no excuse for their mistakes and should be judged just the same as the rest.
And furthermore TOS is
Star Trek. The original, definitive article. TNG, DS9, VOY and ENT are spin-offs made by others decades later.
Voyager references the events of Star Trek: First Contact, as does Enterprise. Does that make Voyager part of this FC/ENT AU also? It crosses over at one time or another with TOS, TNG and DS9, so what does it make those? There's a lot more evidence that all these series' are interconnected than there is that they're separate timelines. Was that meant to be an alternate Riker who testified at Quinn's trial? Another Sulu which Tuvok flashed back to? Or a different Dominion that EMH Mk. 2 mentioned? No way. Just like the Federation which Archer founded at the end of Enterprise is the same one we see in the rest of the series and not some other version - if it was, it would completely undermine the entire premise of the show as a prequel to the Trek universe.
During an episode the title of which escapes me, Picard references Pearl Harbor and "Station Salem One" as examples of "sneak attacks". I really think that if he were part of the same timeline as the events in ENT, he wouldn't have not mentioned the Xindi attack on Florida.
You'd think Data would have known about "The Immunity Syndrome" when the Enterprise came across "a hole in space" in "Where Silence has Lease" (and Riker even asked him, point blank, if anything "even remotely similar" had been previously encountered!)