Bad take, that's fucking dumb and you know it, the common usage of the term relates to the disorder not the mythological character.
we ban calling people a sch*zo here, why shouldn't we ban calling someone a narcissist?

Narcissus is a Greek name. Narcissistic is an english word. The ancient greeks did not call anything narcissistic, because the word didn't exist.

The N word comes from Spanish but people who use it aren't speaking spanish, are they?

Suppose a white skinned american who doesn't speak fluent spanish were to call a black person "negro". When confronted about their obvious racism, they defend their language with "I'm just speaking spanish, and it's not racist in spanish". This is obviously a cope and the person is obviously a racist. While the original origin of the word may be from spanish, the word has passed into english through racialised use. Any use in english by a non spanish speaker must therefore be assumed to be racial.

While the word "Narcissus" has its original origins as a Greek name, the separate but related word "narcissistic" was coined by english speaking psychologists, and it passed into common english vernacular through Christopher Lasch's book The Culture of Narcissism, which presented the thesis that narcissistic personality disorder was becoming more common in comptemporary america. Given that most common use of the word narcissistic is derived from the cultural impacts of this book, it's safe to associate any use of the term in common discourse with the disorder. Especially since while 50% of the users of the word will respond like you did, the other 50% will respond with "Yes, I was talking about NPD because narcissists deserve to be hated for their disorder".

i would just change it to self-centered. this is an online topic that’s not worth the argument and also narcissism unfortunately does have lightly ableist connotations now since the word has now been medicalized

Narcissists are only 1% of the population, yet we see this behaviour from anyone who owns a large platform. Unless you want to present the thesis that people with NPD are privileged because we own all the social media sites, we must conclude that this pattern of behaviour is common to neurotypicals as well.

Nah I think it's moreso the use of the word, but I've only ever known of it by that term; I'm not sure what other label would exist for it.