^In what manner did ENT declare itself free? Simply by virtue of its continuity-defying antics (matter/antimatter propulsion, Xindi attack on Earth that no one bothers mentioning ever in the future, first contact with the Ferengi, first contact with the Borg, random Denobulan, jerk Vulcans, Kronos less than a light year from Earth)?
Because the producers of the show said so. 'Canon' is not a function of the story, it's the other way around. If the producers say "this isnt' part of the old canon", and both Berman and Braga did that, then that, quite simply, is that.
That's why bitching about ENT's continuity issues with the other series was always a bit silly, because on day one, we were told 'this is a new timeline, and you can blame First Contact on it if you want'.