Canon: an official part of the story line of a series, as opposed to fan guesses and dreams. It means it's clearly what the author(s) meant the fans to believe.
As to the comment about Tsubasa being a shonen series, and so being less likely to have a malexmale couple; that's a point, true, but Clamp's X/1999 and Tokyo Babylon technically qualify as shonen, I believe- they're hardly shoujo like CCS- but Clamp had no qualms about putting heavy shonen-ai/yaoi elements in the story. And I don't think any fans complained, considering that those are two of their most popular series.
And personally, I've never seen much in the Tomoyo/Kurogane relationship to indicate romance- just a deep respect and caring. More like a brother/sister thing, to my mind. Protectiveness doesn't mean automatic romantic interest, even between non-related couples. Besides, Tomoyo, to me, seems to be gay. I don't think she's ever been shown to have an interest in any boys, whether in CCS or Tsubasa. Her only romantic interest has been Sakura, a girl. This doesn't techincally mean much- Tomoyo could, of course, turn out to fall in love with a boy. But honestly, poor Tomoyo has always been set up as the one who lucks out- the girl who's in love with someone who happens to love someone else. I don't think Clamp would change that to give her a relationship with Kurogane.