I never paid alot of attention to Enterprise but I just saw the Mirror Universe Episode in it's entirety...
I've heard that people say that this episode confirms that it's the same time line as TOS. But we get to the end...and the Terran Empire is in possession of the alternate Universe Defiant.
That doesn't make sense if it's from the same universe as TOS.
Otherwise TOS would be Far Far more advance than it currently is. The story should have ended with the destruction of the Defiant which would have allowed the Terran Empire to proceed up to the TOS alternate reality.
Now we have an alternate alternate reality...