I do not think it's IQ alone. Maybe IQ score has to do with it after a certain level - if, for example, the ASD impairs self-help abilities and an MR does so too, the person may have doubled impairment in that area. (Uneven profiles with extremely low scores may be causing impairment too.) Beyond MR, I wonder if IQ score is generally meaningless in number of impairments?
As for a high IQ score and the ability to cope, I don't think there is much of a correlation. There are people who score gifted on IQ tests, yet are very impaired in their daily activities, maybe unable to work, cook by themselves etc.
IQ score probably plays a part in the process of development (of course) and copying (sounds likely to me). But it's not the only thing.
A high or low IQ score by itself does not express how well a person learns, how well they adapt to school and work. There are people who're gifted who did average to bad in school or even couldn't cope at all, just as there are people with for example an IQ score of 80-85 who actually had a degree that's hard to attain and are perceived as gifted (not Savants).
Seeing how these cases are real possibilities, there seem to be additional determining factors in outcome than just the IQ score.
Also, what rdos said. The number of symptoms do seem to differ (very much) at birth.
Some babies have immense sensory issues, while others seem to have no big ones or even none. Same with other symptoms.
_________________
Autism + ADHD
______
The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett