Okay, I don't really know much of anything about autism, other than what I've read in this thread. But it seems to me that what the argument going on here is that a raw/paleo or otherwise good diet may help with the symptoms one sees in autistic children. It could reduce problems with digestion/mood, as Tyler said, which could reduce the symptoms of the autism and make it far more manageable, and easier to deal with, which SEEMS like it is relieving the symptoms of autism itself. However, it actually does nothing for the actual condition of autism, instead it is similar to, say, someone getting a cold, and has the symptoms of a runny/stuffy nose, sore throat, and a cough (like the symptoms of arm flapping or OCD-related issues and other things that autistic children do) and then this person takes some Nyquil or other cold medicine (or changes to a different diet). The symptoms of a runny/stuffy nose, sore throat, and cough (or the symptoms of arm flapping/ocd-related issues/other issues) all go away. However, the actual cold did not go away (the autism itself has not gone away whatsoever) only the symptoms of the condition have gone away. But still, you FEEL like the cold has gone away (or like the autism might be gone) but that doesn't mean that it isn't still there. Sure, it's a great help, but it's not an actual cure or even partial cure. I think that it where this argument is coming up from.
Not that I'm trying to get involved into the argument, as, like I said, I know nothing significant about autism, I'm trying trying to show another perspective of the argument from what I read to help those arguing to understand better. If it doesn't help though, then just ignore everything I said, lol.