I get where you're coming from, but there are a few meanings for the word dumb in the dictionary, one of them does mean stupid, which obviously I can't speak for anyone else but maybe that's what they were thinking.
I myself have referred to Deek on here as dum dum, my saying that is because he doesn't speak, and comes across as stupid at times. That's just my opinion, and in no way a slur to any disability. People have different views on things for sure, but the word dumb does also mean stupid as well as someone who can't/won't speak. 🤍