So, autism is a spectrum of common disorders that vary from person to person. Therefore people are not diagnosed on the basis of noted disorders but instead on their performance in a battery of common performance tests.

As for empathy, it too varies from person to person. It is possible, though unlikely, to score high in empathy and yet utterly fail all the rest of the performance criteria. One of my coworkers with autistic children may or may not have autism themselves but does demonstrate high empathy.

In my experience people with autism tend to score remarkably low in empathy with some people even having absolutely no empathy at all. That is why many people with autism seem socially weird or have trouble reading a room. For people with high empathy these observations of low empathy in others is most obvious potential indicator of autism.

While very few people score high in empathy it’s equally rare to absolutely have no empathy at all. It is such a striking disadvantage as to be a major disorder. It is severe enough that it looks like sociopathy minus an informed intent. It’s a processing void. That void is further obviated by an equally diminished introspective capability in that reading one’s self is the same skill as reading others.

Also, empathy is not in any way related the quantity of emotions people display. A person can be both selfish and highly emotional.