common arguments
"what if the person i'm calling a narcissist has npd?"
this is an understandable situation, and to clarify calling someone a narcissist in a non negative way is fine, if it's to specify someone with npd. any problem comes in when someone is called a narcissist as a way to imply it should have the same connotation as egotistical or self centered. npd has discrimination and struggle specific to the disorder, so to disregard it as something else is harmful.
"the definition of narcissism isn't just about npd, people are allowed to use it as such"
a lot of this thinking comes from misinterperting that narcissists don't know how the world views the word. everyone is fully aware of how it isn't inherently incorrect/recognized as discrimination to use narcissist as an insult, however if the person using it in that way is aware of how it ultimately harms people with npd then it is in our right to view it as ableism. there is nothing stopping someone from using narcissism as an insult, but there is nothing stopping someone else from rightfully viewing them as an ableist for it.