Why can Xayah ult while snared?

It's a snare, not a stun. You can still cast abilities and use items.

Xayah ult is technically not a movement ability. As long as the ability doesn’t cause the champion to “move,” it can be used.

its an ability, so therefore isn't blocked by it, BUT the effect of the ability is to become untargetable, thus clearing the snare. think of it like a zhonyas.

at least that's my logic. either way though, to my knowledge, snares only block blinks/dashes/backflips anyway