Autism itself isn't a mental illness. It's a physical difference in the brain structure that manifests as different perceptions resulting in different behaviors.
If those behaviors are sufficiently damaging to the life of the person exhibiting the behaviors, it can be called a disorder. But it's not an illness.
I am autistic. I rock back and forth without thought because it brings the world in focus and eliminates random noise. I often do so to a beat at a half or quarter the tempo of the beat like a metronome. It gets super obvious when I'm a bit tipsy as my threshold for distractions decreases and the need to focus becomes more important.
So what we call autistic in humans might be entirely neurotypical for sasquatches. It can't cause a disorder because it can't disrupt their lives (and they can't be diagnosed, they won't answer the diagnostic questions lol). Their apparent behavior certainly could be compared to the behavior of autistic humans, sharing some behaviors, but it's illogical to tie the two together because different animals have different senses and different neurological makeup.
Calling them autistic is like saying a lion is broken for eating a zebra. No, that's just what lions do.