That’s straw man argument, because a gadol who is unqualified would not perform brain surgery. In fact, I have never heard of a gadol who performed brain surgery (although the Chazon Ish famously mapped out a plan for surgery which a skilled, practiced surgeon carried out successfully).
Part of emunas chachomim is trusting them that they won’t overestimate their abilities.
Well said.
I’ll also add my own theory. Nistakel B’Oraysah U’Bara Alma. Someone who is so engrossed in Torah that he only takes what the Torah thinks into account will naturally have a deep understanding of the physical world as well.