this would be because xin's Q is actually still a channeled ability like it was back when he was created. as for why it was ever made this way i have no idea, but there is video proof of this back in the day where you can literally see the channel of the ability.
Xin's W and E also function this way on a fundamental level as well ( mostly his W) in which you can interrupt your auto attacks by using his spells which quite actually hurts xin quite a bit if you dont know this about him.
For example, it is highly possible for you to begin an auto attack, use your W to interrupt said auto attack, then use your E to cancel the next auto attack, and then use Q to interrupt it once more.
All of this can lead to xin missing out on a big portion of damage and also has a slight hidden skill cap if you are not aware of such things.
As for your statement on the Q itself though, with enough attack speed, it is highly possible to actually keep xin rooted in place unable to preform any action at all because his Q will keep going off and keep using the channel. this has actually caused me to lose many duels in which i would easily win if i could just sidestep any sort of ability while not being rooted in such a way.
Xin overall also has other problems. such that his Q does not proc additional lifesteal from the bonus damage of three talon strike, or that xin's E STILL has a shorter range on it that it shows ( which has actually been in game since xin has been in the game) or the sometime odd bug where you can ER so fast that you can actually knock back your challenged target.
to be fair though, if all of this was to be fixed at the same time, xin might also become even more of a nightmare at the lower brackets ( which is why xin can hardly see competitive play, his numbers cannot be good enough for it because his kit is too easy to work with and become a monster)
if i had to make a list in what order these bug fixed should go through it put it at this
- Xin Zhao's Three Talon Strike's bonus damage also applies lifesteal effects.
- Xin Zhao's Battle Cry and Audacious Charge no longer interrupts auto attacks.
- Xin Zhao's Audacious Charge has had it's hitbox corrected to match the visual.
- Xin Zhao can no longer sometime knock back challenged targets.
- Xin Zhao can now properly use all abilities and move during his Three Talon Strike.
These are how i would list as major changes which effect you more often than not / can be incredibly devastating if it happens to you ( which is why numbers 4 and 5 are where they are, nothing says FUC U! like pushing away that assassination target while you just sudokud into the enemy team just to not even get a single change to kill your target/ CC it down.
