Yas Windwall not block Xerath Q

I'm pretty sure its meant to do that, his Q is not like other skill shots where its one thing goes from point A to point B.

This is intended. Xerath's Q has no travel time from it's start to end points, so it's not considered a skill shot.

Xerath Q is not a projectile, it's a ground targeted that applies all it's damage at once. Yas windwall only blocks projectiles, so the only thing it blocks for Xerath is his E, the stun.