yi q during stun

He might of que'd it before he was stunned.

Nah I think what he did was a yi mechanic/bug called disjoint. Its a clever way to mitigate hard CC (stuns/snares) with yi. Just alpha right as the stun hits to disjoint the ability, the hard CC must land on yi for it to work.

See below video for a better explaination, especially around the 40 second and again at 2:52 second mark. https://youtu.be/heneW2EdGzs

Note: Dodge = avoid damage and CC Disjoint = mitigate (reduce duration or effectiveness) of CC but take the damage.