Why do people tend to say the Kara no Kyoukai anime "does not exist"?
Kara no Kyoukai, like other Type-Moon series exist in as part of the same multi-verse. While not in  the same universe as Fate or Tsukihime, they exist as an alternate universe, like Fate/Extra and Fate/kaleid liner. The characters could in theory meet one another... if the circumstances are deemed right.

Canaan, however is not part of the this multiverse.