There's no flaw in saying, "Don't defend transphobic people". Transphobic comments are terrible. Period. Don't repeat them. Don't "what about..." them (like you just did) Don't defend them. It's that easy because everything about transphobia is wrong. And that you think that there is a question about if mutilations is a hurtful word is very telling. Especially after being told repeatedly that it's horrible.
Just like racism or homophobia or Islamophobia, there's literally nothing to make up your mind about. They're just people living their lives and if you treat them differently or start trying to add equivocations to how they should live their lives then you're a terrible person.