Those who claim that positive thinking can affect illness recovery have the burden of proof, because they are making the claim.
The only knowledge you need is what constitutes sufficient evidence. PG's n=1 anecdote of how he thinks he recovered certainly doesn't, it's far more likely that he was going to recover anyway and couldn't resist giving himself a pat on the back for bringing it about. He might as well claim that having a smug self-congratulatory attitude was the key to his recovery, or a new brand of tea, or whatever.
Any time positive thinking has been subjected to proper scrutiny it has not been shown to have any effect on recovery, for example here:
https://www.apa.org/monitor/jan08/cancer
And many other such studies to be easily found by googling "positive thinking cancer recovery study". Not only is positive thinking ineffective, it is also a common tool of abuse not only in medicine but many other fields. For example if a multinational corporation wants to lay off 5000 workers, it sends them all on a positive thinking course 6-12 months before the announcement. Then when the announcement is made, instead of unions taking to the streets, they have 5000 former employees who have been conditioned to see opportunity in adversity and go off with plans to thrive and prosper. Gets the company off the hook.
Until some disinteresting party conducts a methodologically sound trial to prove such a proposition, I have no reason at all to believe such a thing. I don't have to claim or prove it to be impossible. Those who make such claims often have something to sell, or an ego to boost, or a belief system to support, or some other conflicting interest which means their claims should be taken very sceptically. PG is no exception to this.