Lol, first, lots of english words have their origin in greek or roman historical figures but few of them actually bear any resemblance to their origin in contemporary usage
second, the common usage likely came from the fact that Narcissus was in love with himself, not that he was asexual
Casting the usage of the word ‘narcissist’ as being an intentional reference to a marginalized sexual preference instead of it’s common and even documented definitions is a bit overzealous