I'm not sure why you think I disagree with your comment.

You presumably don't, but it contradicts what you said. (See below.) I expressed the additional information because it makes the conversation less wrong.

Taking a step meta is generally acceptable. People claim the Holocaust never happened is not taboo, even if The Holocaust never happened is taboo in many contexts.

If you look closely at the actual information communicated you may note that this qualitatively different. Your example is of an entirely meta claim being made. The case in the grandparent is a meta claim being made as a prefix to an actual object level claim. Specifically:

I'm not 100% sure that is true, but I think it is true. At least, I know I did and I believe that many children engage in a fair amount of self-stimulation.

An analogous "holocaust" claim would therefore be "People claim the Holocaust never happened. I'm not 100% sure that it never happend but I think it didn't. At least I know all the evidence I've encountered is faked and I believe that a relevant general class of people faked evidence."

I think the ancestor is a +12 because it is a great example of what the OP requested - a true, probably taboo sentence.

I agree entirely. That it also happens to be an additional way to actually make claims about the subject while not triggering the taboo penalties is secondary. And incidentally an example of a generalizable social tactic.