I appreciate the example and I think I see your point. I agree with the underlying logic, in general, but applying it to the N in NPD seems an over extension.

Dictionary definitions for the two terms, as records of common usage, are notably different. Autism refers solely to the condition so your example sentence would be an inappropriate use. Acceptable and understandable in the language, but an uncommon application of the word. On the other hand, narcissism is used for general egoism and self importance first and for NPD second.

This of course doesn’t invalidate your feelings when hearing the word or desire to protect others from the same, but maybe this can offer some comfort if the most common usage is not intended or even understood as a slur or even a reference to folks with NPD.