Can you provide sources for this? Like, how many people thought chess or go were literally impossible for computers to play at superhuman level? I'm actually curious about the history, and I don't want us to accidentally attack a straw man.

To me, the fact that Turing wrote the first chess program in the 1950s suggests that people thought computers were capable of playing chess. It doesn't seem too great a leap to imagine that they'd be open to the idea that a supercomputer could play superhuman chess.