DY: That’s more of a problem, then. How could Chazal be Chozer if their original Shittah came from a Mesorah from Sinai/Torah Sheba’al Peh? How could they have been wrong in the first place?
I don’t know why ??? ?’ ?????? would need to be from a mesorah; Hashem has many ways of revealing things, whether through experience, speaking to contemporaries, drashos, sevaros or Ruach Hakodesh. The only guarantee is that the end result, as stated in Mishnah or Gemara, is accurate.