What Makes Xayah's Ultimate Different from a Zhonya Stasis?

So apparently Xayah's ultimate not only makes her untargetable but also invulnerable, something that the description has left out.

I learned this when I was able to negate Yi's Alpha Strike damage by timing her ultimate just before his alpha strike finish completing its animation thereby nullifying its damage because alpha strike damage doesn't register until he reappears next to Xayah.

You could think of it as a mobile Zhonya stasis that last shorter. However, is described as making its user untargetable AND invulnerable but 's ultimate is only described as making her untargetable. Why is that?

[zombie-brand-mindblown]

If a 4 second DoT spell is applied on Xayah, will she suffer the DoT damage during the 1.5 second she was untargetable within that 4 second DoT time frame? By using the logic that Alpha Strike's damage is negated while she's in that "mobile Zhonya statis" form, she in her ultimate form also shouldn't suffer damage for 1.5 second out of the 4 seconds DoT duration, right?

[zombie-brand-mindblown] [zombie-brand-mindblown]