That's a massive exaggeration and you know it. Shinnok can cancel his low into hellsparks to stay plus and make you guess again. I agree Quan is better but Shinnok, in Impostor, has his fair share of dirt on Quan which makes it a 5-5 imo.
I don't think it's an exaggeration. Impostor Shinnok hits you and his vortex ends after one loop depending on the special he steals. Boneshaper has a fuzzy-able mix up, with the OH option being unsafe and the throw ending the vortex, low option ending in hell sparks giving him another mix up as you said. This always seemed manageable, in all my MU experience against him he never seemed "broken." There's a fair chance to block Shinnok's mix ups.
Quan can do the same things, with important differences. In any variation, Quan can loop his vortex with no limitation to trance, unlike Impostor. The mix up following is unreactable, a true guess, again unlike Shinnok. Also, when Quan spends a bar, he's so much more advantage. Then there's the obvious; once Summoner Quan has you, the likelihood of blocking the bat/ OH is so slim that the smart money is on the round being over, every time. That's just scratching the surface.
Maybe new tech will come out, making bat easier to block though. Not crying for nerfs, just stating the obvious (?) differences.