I do think they're doing a "heads I win, tails you lose" style argument where they say calling someone colloquially a narcissist is inherently ableist because both it's not medically sound terminology a lot of the time but when it is medically-backed it's not allowed because they named a medical diagnosis after a pre-existing social phenomenon. So there's a conflation in terms happening here.

But they are also correct its dominant current usage is "person i don't like" with a vague air of being legitimized by its association to a medical diagnosis. I'd liken it to how right-wingers call every one they disagree with mentally ill, which we can see is definitely ableist.