I'm a descriptivist but that doesn't mean that there aren't rules and that we can't point out things still being wrong.

Descriptivism still describes rules as they're used in the real world. Breaking those rules still subjects the speaker/writer to the consequences: being misunderstood, having the spoken or written sentence to simply be rejected or disregarded, etc.

"Colour" and "color" are both correct spellings of the word, because we are able to describe entire communities who spell things that way. "Culler" is not, because anyone who does spell it that way is immediately corrected, and their written spelling is rejected by the person who receives it. We can describe these rules of that interaction as descriptivists, and still conclude that something is wrong or incorrect.