It's cannon, though it doesn't feel exactly right.

Like, Kyousuke wasn't extremely happy with Hitomi confessing to him, and he'd probably accept most confessions, if Sayaka would've confessed he'd accept it. (if you're 14 and you got 2 girls crushing on you, then you'll be fine in your dating life unless genetics ruin it)

I honestly feel bad for Hitomi in this situation, sure she "stole" Sayaka's man, but it wouldn't have done anything. Either way, Sayaka would've fallen into despair. Hitomi doesn't know what Sayaka's going through, if she did I'd doubt she'd do such a thing to her.

But Kyousuke and Hitomi are trying their hardest to work things out around their tight schedules. Hitomi is frustrated and Kyousuke's clueless.

So I guess the ship's fine. It isn't the best, but it certainly isn't the worst.