I am not confusing them so much as avoiding trying to write a textbook on autism when addressing the original issue. I was sloppy. I may not have succeeded at my goal of not writing too much.

The existence of mirror neurons is not just a theory. A mirror neuron is (by definition) a neuron that fires both when you do something and someone else has done something. Those are observed. It is a theory that it causes autism, but not a theory that they fire less in autistic children, or that these areas of the brain are almost precisely the same places where autistic spectrum people have thinner brains then controls.