Sod Hashem Liyreiov doesn’t mean He tells them every sod, but He does tell them the facts needed for a correct psak. If they figured it out through their own science but got it wrong, He sent the chachmei ho’umos as shlichim to give them the right information.
You don’t need to ask a kashya from chachmei ho’umos; if individual members of Chaza”l were infallible, how could there be a machlokes amongst them in these inyanim?
The teirutz is the same; it’s not that individual chachomim are infallible; as great as they are, they’re still bosor vodom. Hashem, however, doesn’t let them err when it would cause an error in halacha.