Ultimately, whether or not something is a disease or disorder isn't a matter of scientific fact, but is instead a social value judgment on the desirability of the characteristics at issue. In the book Unstrange Minds, cultural anthropologist Roy Richard Grinker makes this point quite clearly (page 11) when he states that "autism, like all disorders, does not exist outside of culture. It is culture that sees something as abnormal or wrong, names it, and does something about it..."
Throughout history, diseases have been defined as a matter of popular consensus. Often, as with cancer and influenza, it was based on simply observing that people were dying or in serious pain. In other instances, cultural beliefs led to the popular acceptance of such things as demonic possession and witches' curses. But although modern medical science has studied many conditions in great detail, identifying their particular characteristics to an extent unimaginable in more primitive times, the underlying determination of whether or not something is a disease is no less a cultural one.
So there's really no way to prove by scientific means that autism is, or is not, a disease or disorder. The argument against using language based in the disease model is fundamentally one of ethics: Should a group of people be labeled as disordered if they themselves do not agree with that characterization? This is the same argument made a few decades ago by gay rights activists who successfully argued against the medicalization of homosexuality, and society ought to respond to it in the same way.