QSS doesn't work on malph ult?

So I just played a game with an enemy malph who liked to ult me on sight. I had a fully built mercurial and used it mid-air after he ulted me yet, I still floated in the air for that half-of-a-second during his ult.

Being knocked airborne is CC soooo...why can't I cancel it with mercurial when it's supposed to remove all cc effects? Maybe I'm missing something?

PS. I made sure to use mercurial after his ult landed and cc was applied.