I think the phrase should be “context-appropriate”. Flapping, or any involuntary stim is just that, and I think the context of the action is the determining factor. There are many “NT” actions that could strictly be termed stims (e.g. nail biting, finger strumming, foot tapping) and all are what I believe to be simple automatic muscle movements which the brain does subconsciously in certain situations just to ensure the muscles are actually there! – ALL of us do it. I think that the difference in the ASD brain is, as in many situations, the automatic context control filter is out of adjustment – to a greater or lesser degree.
Flapping was said to be “a sign of a poor proprioceptive system”
I’m not so sure that stim actions are part of the proprioceptive system, more of a perceptual action in my mind. While the dyspraxia and general clumsiness in ASD is clearly proprioceptive, I think stims are more of an intentional (subconscious) action which an individual seeks to ensure sensory motor feedback and finds it comforting. I don’t think this somatosensory processing is anything other than an extension to other sensory processing differences present in ASD.