The unnerving, unbalancing effect Fischer had in the 1971 Candidate's tournament was absolutely due to his play. He put up immense resistance against lines prepared by teams of Soviet GMs (example here https://www.youtube.com/watch?v=X8-90cPf61M), which was the sort of thing that broke their nerve. He was not intimidating or manipulating his opponents outside of the board; if there were any outside-chess forces at play, it was the pressure from Soviet higher-ups embarrassed to lose to a lone American. Also, I don't recall that his opponents played worse than they would have been expected to against an opponent so much stronger than them — maybe after their fifth loss in a six game match, they'd become demoralized.

This is nothing like the Carlsen-Niemann case.

In this case it seems most of the unusual behavior occurred after the match, where (I've seen claimed) Niemann gave obviously wrong reasons why he had played so well on such an uncommon line, and was also not able to explain why he made particular choices he did playing it.

We can be reasonably sure that Fischer has not been cheating above and beyond unusual conduct. There hasn't been a way to meaningfully cheat. In our times however there are computers which can materially help, and there are technologies allowing someone to receive it, use of which is very difficult to detect without an unacceptably invasive search