More to the point, we have no idea whether the events of "Relativity" are themselves occurring in an alternate timeline because it's only discussed subjectively.
In other words, while they exist in a timeline in which the Borg interfered with "First Contact", we don't know whether it was the Borg of their timeline that interfered or whether it was Borg from a different timeline that did so. And if you believe that when you travel into the past you jump tracks, then the latter must be the case.
Any of the possible results are possible. It won't always necessarily be an "alternate timeline is created when you travel back in time" in Trek, it won't always be "possible to change the past" nor will it always be "Impossible to change the past". Any of these three possibilities may happen when Trek does Time Travel, we've seen examples of all of them.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?