While it's true that autistics have difficulty reading the facial expressions and body language of neurotypicals, autistics often have considerably less difficulty reading the facial expressions and body language of other autistics. In fact, autistic individuals usually find reading autistic facial expressions and body language easier than neurotypicals do. There's a pretty good first-hand account of the sort of thing that I'm referring to here. The comments on that are pretty eye-opening, as well.
On a similar note, a discussion of ABA on the ASAN's discussion list included the following anecdote (reprinted with the sender's permission):
A neurotypical teacher (who gets along with me just fine) was watching me doing some academic testing a couple of days ago with a student who is classic autistic, and she had to repress her laughter while she watched... because suddenly I was speaking a totally different social language (as in, being the real me), and she later said it was "wierd" to watch me "lose all my feelings," and yet he and I were clearly playing around and having a great time (the kiddo was laughing quite a bit, especially when I called him on some of the get-out-of-work-free behaviors he had learned because people don't normally understand him and like any kid would, he was playing it to the hilt). It baffled her. Especially when he then started giggling uncontrollably after he discovered that I was very good at eye-contact tag (letting him lead and just giving the briefest of glimpses at each other's eyes while I played "hard to find" by making him work to "catch" my eyes... he was intrigued by an adult that didn't force eye contact and then would actually dodge it to create a game).
That led to a rather long conversation about this very thing... that there are multiple social langauges, including the toned-down version.
There are similar issues with people from different cultures (For examples, I suggest studying the initial encounters between Americans and Japanese following Perry; I believe there were similar accounts involving the Chinese, but I haven't studied them in enough detail) as well, so I ultimately believe that it's not an issue of autistics having difficulty reading body language (and/or facial expressions) as I believe it's a matter of people in general having difficulty reading body language that's different from their own.