I'm assuming you meant to write ableist, in which case, how is the above comment ableist?
The funny thing about this is that I simply read “dumb” as “stupid” and didn’t make any connection to being mute - but thanks to the earlier comment I’ve now made the association. So, well done policing the discourse, I guess?
To answer your question, adjectives related to intelligence are ableist in origin. I don't think this needs to be explained and it's not my job to educate you or anyone else on this forum.
I doubt driveby condemnation will convince many people.
moot since the comment itself is dead/flagged but if you're calling people idiots, show me your credentials.
I think that's because without the explanation it's more or less name-calling, since the accused doesn't know what they did wrong.