As the title says, he is rooted but still is able to q.
I know he casted it just about when he was rooted, but as far as I know, you shouldn't be able to still be rewarded with an ability to execute if you don't time it well, especially if you're playing a champ as easy as Yi.
Ahri ult while rooted? Impossible.
Kayn Q or R while rooted? Impossible.
Camille Ult while rooted? Impossible.
Nocturne ult while rooted? Also, impossible.
Why should Yi still be able to do this?