No, Lulu cannot stop Sion's ultimate. Stop lying to the boards.

EDIT: This has been fixed. It was never an intentional strength of Lulu's, and so was never something that should have been complained about. If you didn't notice that this never happened before, and there wasn't a patch where they intentionally forced it to do this, that's on you.

This is the video used as "proof" that she does.

Some problems though:

1. The point at which the polymorph spell actually reaches Sion is the very end when he'd already queued up the slam-- past the point where he's immune to CC. You can see this by the fact TF's gold card also stuns Sion within that same second.

2. Neither form of CC actually prevented the slam at the end of his ultimate. He simply missed. Sion only knockups up enemies he collides with. That Jinx is well out of collision range, and as such she is only hit with the damage and the 35% slow. Nothing prevented the knockup except for Sion's timing and aim. He attempted to end it so that he would still hit Jinx instead of colliding with the dead turret. He did succeed it hitting her with the damage and slow, but was quite far from collision with her.