Can a 6yo with autism be advanced in math?

Absolutely. Recall dustin hoffman in "rainman"? This phenomenon is not understood. Outstanding skill in music is equally poignant. There are others. The victorians labelled them ''idiot savant", the knowing or knowledgable to a very impressive extent existing in an individual who was equally constrained interpersonally, given to repetitive behaviors, and extremely distraught if a routinized behavior is disrup.
Splinter skills. Some autistic children have what are called splinter skills. As a result, some have extraordinary skills such a omputer like math skills.
Yes. Autism has nothing to do with intelligence. Iq scores on autistic children may not accurately reflect their innate intelligence potentials. I think autism reflects more sensory processing challenges.
Yes, until later. Educational demands require conceptual thinking for math applications. With an over-developed rote memory & a Non-Verbal IQ > Verbal IQ, students with ASD can do very well with math computation, even if they don't have "splinter skills". Later, deficits in language & the ability to grasp concepts that cannot be seen, heard or touched required for "word problems", etc., make math tougher for them.