“Cis is a heterosexual slur” Tweets Elon Musk, Who Halved Twitter’s Value — Assigned
(www.assignedmedia.org)
That's a technical argument. Technical arguments don't work on transphobes because transphobes aren't motivated by logic, they're motivated by emotion. The correct argument to use on a transphobe is one that humiliates them in the eyes of all onlookers. The only way to reform a transphobe is public shame.