I'm not saying it isn't correct. I'm just saying that it's rather nonsensical to single out Enterprise and stress that it happens in an altered timeline, when pretty much every Star Trek production since the first instance of time travel in Star Trek happens in an altered timeline. What's so different about Enterprise?
NCC-1701:
Also, I was thinking about this some more, my friend. And I came to another conclusion, as well. The changes within the time lines you mentioned or within all of Star Trek's episodes have been very minor or they have been corrected (as if they didn't happen).
So just what type of significant changes could there have been within "Tomorrow is Yesterday"? Kirk and crew are spotted by the government and regarded the whole thing as a UFO incident that was essentially covered up.
In the City on the Edge of Forever: there was no changes to the time line at all.
http://www.tubechop.com/watch/62381
Should there have been any temporal incursions for Assignment Earth? Kirk and crew didn't come into contact with anyone that would have drawn suspicion that they were from the future. They didn't leave any technology behind or tell them of the future. Plus, Gary 7 was there to preserve the time line.
As for All Our Yesterdays: It was a time travel event that only effected that one particular planet that was not a part of the Federation. People of that world simply went back into the past to avoid the destruction of their world.
Should there have been any lasting effects within the time line for Star Trek 4? Maybe. But seeing the time line wasn't changed suggests otherwise. So apparently, the Assistant Director of the Cetacean Institute (Dr. Gillian Taylor) was not that influential of a person past 1986, either due to the fact that she might have died or disappeared from the public eye. Also, when the whalers see the Klingon bird of prey, they didn't take any pictures of it. So there is no proof that anyone would believe them. And of course, there wouldn't be any lasting changes within the time line if two humpback whales went missing (especially if they were going to be slaughtered, anyways).
See where I am going with this?
Pretty much all of the events we seen within the various Star Trek episodes have addressed the small changes within the time line or they were so insignificant that that there was no noticeable effects later on.
In Star Trek First Contact: We have a major influence of the time line by Picard and crew. Cochrane and Lily learn of a future world and technology that they didn't know existed (which was set into motion by them). They learn of a deadly race known as the Borg, too. The difference with this incident and the other time travel incidents with Star Trek is that this time travel event should have had a major change within the natural development within the Star Trek Time Line. But yet, we don't see any noticeable changes within the time line when we later see the 24th Century. Which suggests that there was some type of correction within the time line or we are seeing the Original Time Line (that can't be destroyed or allowed to be changed by time).
However, that doesn't mean there wasn't a change to the time line at one point or that it didn't create a separate alternate diverging time line, though. If we were to follow Lily and Cochrane after Picard and crew's little intervention I am sure they would have tried to push for advancements in humanity a lot faster than they originally would have (to either protect themselves from the future Borg or because they simply now knew that the technology exists).
So the difference with Enterprise is that it fits the mystery of the strong influence on the time line we seen in First Contact. It was always suggested in previous Trek canon that the technology was more primitive within the 22nd Century. So seeing Enterprise shows us a more advanced version of that history, and it confirms itself to be within the First Contact Time Line (within the episode "Regeneration"), it makes sense that we are looking at an alternate or changed time line.