Tenacity doesn't work, apparently [GAMEPLAY]

EDIT: People are telling me that knockups aren't affected by tenacity. Which is true, except in the tooltip it says it's a stun. It does knock me up a little bit, but look at the video. I'm in the air for only part of the time, and then I Iand on the ground again. I'm not even in the air for the whole thing.

I was playing a game just now against a Sion. I have the tenacity mastery that grants 10% tenacity (minus 10% duration of stuns and such). I have the replay in the link above of two instances where Sion hit me with his ult from max range. With 0% tenacity, this should stun me for 1.75 seconds. But in both clips, it stuns me for 1.75 seconds even though I have tenacity. You might be thinking that I'm just thinking that, and 10% off of 1.75 seconds isn't noticeable, but my tenacity actually CHANGED between clips. You'll notice in the second clip that I have Mercury Treads, which brought my tenacity up to 40%. In both clips, I timed with a stopwatch that I was stunned for 1.75 seconds exactly. You don't need to time it yourself. You can see in both clips that I just barely get out of the stun right as his Q smashes down. That shouldn't be the case if my stun duration was reduced more in the second clip. So it wasn't.