The rule has always been "episodes that aired." Partly that's for convenience, but partly it's for accessibility -- not all fans will even be
aware of bonus content or deleted scenes, which is why My Fave Dude
Martin Madden is (to my endless disappointment) still not canon. The canon is defined as the body of work released as regularly-packaged episodes known to all, and it would be best for the fans if the definition stayed that way.
It is very obviously the case that the facts revealed by this scene are going to become canon soon enough, but it isn't canon until Season 2 starts up and reveals it. For now, it's just a teaser trailer.
I know I'm a stickler, but recklessly labeling cool stuff "canon" is how you end up with the garbage fire of comic book continuity (and comic book fandom, which is somehow even angrier about everything than
Star Trek's).
(Plus, obviously, I'm in the minority on this. Memory Alpha policy always wins all canon discussions, and deservedly so, so I suppose I should go make my case there rather than here.)
EDIT: BONUS ROUND: are title sequences canon? I once had a serious debate about this, since
Enterprise's title sequence has a little history hiccup in it -- they show the space shuttle
Enterprise in orbit, even though the real-life shuttle
Enterprise never flew. I tend to think that they are, despite the complications this causes. ("One day Captain Sisko went out on a runabout and was just flying in front of the station on a beauty shot when he was startled to see the words STAR TREK DEEP SPACE NINE appear floating in empty space just behind him.")