Something odd about Malphite ult [knockups in general?]

I was just playing against a Malphite, and he killed me twice in lane by ulting me and bursting me with his combo. Which isn't inherently wrong. .

What was wrong was that, after landing from his ult, I still couldn't use any spells. I would have hit him with my CC, and fucked him up instead, but even after hitting the ground, I still couldn't use any spells.

Fine, "I pressed the button a little two early," but you'd think that after the third time I hit the key, I would have been capable of using a spell. It's as though, after landing from being knocked up, I was under a half second stun at least!

It's one of the things I'm disliking about this game: What's happening on the screen is not representative of what the characters are genuinely capable of doing.