Hi Autisticskeptic! Welcome to Wrong Planet! Check out the many interesting and helpful forums here. You are among friends here at WP!
Yes we do have mental processing problems, and so do those with Down's Syndrome. However, there are some big differences, including chromosomes involved, so they are not the same. Autism includes a large number of things that are not part of Down's Syndrome. Also, a large part of those on the Autism spectrum are not intelligence type of mentally impaired. There are some people on the Autism spectrum who also have Down's Syndrome, but the same occurs in the rest of the population, so don't see this as the same thing, just because of one ignorant doctor or therapist.
Hope that helps. ![Very Happy :D](./images/smilies/icon_biggrin.gif)