"Narcissist" is not a slur. Additionally, he's calling you out for your bad behaviour and online harassment, not "because you have NPD".

If you have a mental disorder that prevents you from behaving nicely online, it may be wiser not to comment. Because people won't see your NPD as an excuse, but as a justification showing that you do demonstrate bad behaviour and therefore are prone to breaking the (social) rules. Ergo justifying the ban.