This is something he mentioned literally a few times between 2003 and 2013 (and he talks about a lot of stuff - check stallman.org) and he doesn't even believe anymore. In 2019 he said that it was something he used to believe, but he realised he was wrong. You can read it on Wikipedia (unfortunately I can't find the original source for the quote right now): https://en.wikipedia.org/wiki/Richard_Stallman#Controversies