Yes, the poster is offensive. That's the joke. See, Scott Adams (the guy behind Dilbert) once wrote that there are six elements of humor, and a joke needs at least two of them to be funny. The more of them the joke has, the funnier it will be. I haven't been able to find a solid counterexample yet (and the first thing I did when I found the rule was try to prove it wrong by thinking of a seventh element or a joke that succeeds using only one). The six elements of humor are witty, naughty, cruel, bizarre, recognizable, and cute. The poster at start relies on being cruel (obvious) and bizarre. The notion of autists just being a**holes with a diagnosis is bizarre, and this is the crux of the joke.
Of course, the joke can work just as well by swapping out the "bizarre" element with the "recognizable" element...But so? It works off of whatever belief the person already has, and doesn't do much to propagate anything. Besides which, what's the audience for this poster, anyway? The number of people who'll see this is probably no higher than five or six digits (keep in mind, most nations have a population of at least 20,000,000), and that's being generous.
I'm all for being wary of threatening propaganda campaigns against any group that can't easily defend themselves, including the one I'm apart of, but this piece of "propaganda" simply isn't threatening.