This becomes relevant every now and again, when people decide to have a problem with people who need more significant or obvious communication supports. The stars have aligned & I can write about it at a time that it's relevant to the broader community.
One of the things Autistic people know that doesn't make it into the literature is that we have difficulty with movement. Not necessarily clumsiness (though that is common), but inertia. Stopping is hard. Starting is hard. Executive dysfunction to a rather extreme and annoying extent. So, we have people who need support for typing, some members of our community are very prompt dependent, et cetera.
But a lot of us need to borrow praxis or initiation. It's a Thing. On a fairly regular basis my roommate & I will be standing at the top of the stairs and one of us needs tea or coffee, the other needs food. So whoever is having more trouble getting started will ask if they can come down at the same time. Or we both are stuck & need to go at the same time to borrow, so to speak, the other person's intention to go down the stairs. Thus we both go to the kitchen.
Am I making her tea? Is she making my macaroni and cheese? An awful lot of the arguments against supported typing go with "well, the supporter is the author of the communication". Someone standing behind someone else, touching their shoulder, is hardly showing evidence of writing with the other person's hand, any more than my roommate is making my mac & cheese because her movement helped me initiate my movement to get to the kitchen. Or maybe I'm making her tea.
No wait, that's ridiculous.
I don't even think this is autistic-specific phenomenon, though we are more likely to have to borrow others' movement. There's all sorts of psychology literature on how people perform better and easier among liked peers. They may not actively need a trusted person around to communicate clearly, they may not require someone else's momentum to get down the stairs (I keep saying that because it's my actual existence), but all people benefit from others. Study groups are basically this exact same thing, on a cognitive level (until they turn into socializing with a pile of books present).
Being skeptical of supported typing requires being skeptical of a whole lot of movement and getting shit done that I do, that many of my friends do, that non Autistic people do, is the point here. Taking a kick start from other beings is a thing we all do. It isn't valid when able people do it, sort of valid when people with fewer support needs do it, and non valid when people with more intense support needs do it. It's the same thing regardless, & it's valid or it's not valid.
And if it's not valid, I've been making an awful lot of tea with someone else's hands.