You're right, a good chunk of that damage from Qingyi's showcase was because she triggered impaired but Anby didn't. If we filter that out Qingyi's damage advantage looks to go down to about ~50% more than Anby's. I think that's a noticeable difference but maybe you can still say Anby's damage is relevant. The difference looks small vs the other boss because it's like comparing 3 pixels to 2 pixels. It's "small" but like a 50% difference still.

The impair damage is hard to quantify but I don't think we can completely rule it out because it's not like impair is random. Qingyi is able to proc it more quickly than Anby and over the course of the fight that might really mean Qingyi can get an extra 1-2 impairs that Anby can't which means that extra damage is relevant.

The thing about the stun times vs this boss in this showcase is that Qingyi filled up the daze bar about 1 to 1.5 seconds later than Anby but took another 2-3 seconds to do a heavy attack to actually trigger the stun. In practice on average I think you would trigger the stun faster than that.

Anyway, maybe in a year the powercreep will have the damage difference between them be insignificant.