Nonsense. Krillin and Tien could fight Saitama evenly, even Roshi as of Super's revelation that he's gotten back into the game, but Yamcha is the weakest of the humans.
Saitama can't beat Yamcha. Saitama's superpower is described as being able to defeat anybody and anything with one punch. Yamcha is nobody, so he can't be defeated like that.