So is your claim that increasing that increasing the chances that the child will be able to fit into adult society doesn't count as a good reason?

My claim? The grandparent doesn't make any claims about autism or the optimal development strategy for those with particular symptoms. It describes claims already made and draws conclusions about whether "straw man" can apply.

Doing things that are abhorrent to people for reasons is still usually torture. (Sometimes it might be self-defense, or surgery, or something.) Stopping people from moving around for reasons is still usually restraining. (Sometimes that is self-defense, or protection of your privacy, or something.) The claim that these measures will help as you describe require support, but even if you could demonstrate strong reason, there would be reason to be suspicious of this kind of therapy!

If the kids involved were not autistic, and the torture/restraint were something corresponding to allistics, you would never get approval for human trials. ("I'm stabbing my son with this thumbtack repeatedly for ten to fifteen minutes every day. He has a really low pain tolerance, so this organization I found says that that will make it hard for him to function as an adult - I mean, he'll still have to show up to work if he has something like a broken toe, right? - so they recommend this intervention." "I don't let my daughter out of her room. Ever. It's okay, she has an ensuite bathroom. When she grows up she'll probably have an office job, and she'll just have to get used to not being able to run outside and play or get herself a snack or anything.")