No, you’re not understanding. There is nothing saying, RAW, you can’t play as a gnoll. The RAW is that you can’t choose a gnoll as your race - but in the case of being polymorphed into one, you didn’t choose that, you were changed into one. And the RAW for that is to assume that everything works the same as before, barring what is explicitly said in the spell for that transformation. And once again, nothing in the spell says you stop playing as a character who is transformed, regardless of if the transformation is “playable” or not. There is not even a “playable” tag for anything, anywhere, in RAW.
If that’s not what you were saying, then what were you saying?
They are not. Subconscious bigotry or bias doesn’t necessarily make one a bigot, as contradictory as that may sound. Whether they like to admit or not, everyone reacts differently to people who are noticeably different from them. There is no way to entirely avoid this, only to realize when it happens and not act according to it. Or in other words: “I don’t care if you’re racist! EVERYONE’S racist!”