IMO the heart of Autism is the social disabilities that impair interaction with other humans. In our species functional interaction is critical to survival, therefore social inabilities = survival anxiety. Constant survival anxiety cannot help but lead to some outward physical expression, thus if one has Autism at a diagnosable level, some type of stim is almost inevitable.
However, as the DSM states, a stim is a 'body movement' - rocking, swaying, hand flapping, etc, NOT hair twirling, finger drumming, nail biting, nose picking or any other small nervous fidget. All humans do those little things. Only select humans sway and hand flap constantly.