I think when people equate the two they're thinking more about vulnerable/covert narcissism than grandiose. The personality disorder diagnoses is generally based on grandiose narcissism though (and it also has to occur not as a part of another personality or mood disorder like bipolar. Like Kanye West is a narcissist but he probably doesn't have NPD because he has bipolar.)
If you reduce narcissism to being self absorbed then it's true of every personality disorder I think. Can't think of any that aren't actually off the top of my head. Even the schizo-spectrum ones (SA is likely on that spectrum as follows: SA > AvPD > Schizoid > Schizotypal > Other schizophrenia conditions) that involve diffuse identities still involve some level of self absorption or self absorption as a defence from losing the self into others etc. If you're very sensitive and you absorb everything that's going on around you because you have no filter for information - all the sounds, and the emotions of other people. Like this image
it starts to drive you crazy especially as you become more stressed or experience certain negative life experiences that are overwhelming. This is a bit different to basic narcissism but like I say most people with one PD have symptoms of others, they all overlap, they all have fuzzy borders until you get to the extremes etc.
Cause like most disorders are tied to problems with theory of mind, emotional dysregulation, stimulus processing, and self-other distinctions at a base level.