If the person knowing the passcode is around and you can fool them into using their passcode then yes, you could capture their passcode. Touch ID is even less of a problem because taking someone's fingerprints is a lot easier than taking a passcode out of their head.

But in both those situations the weakness is in the person, not the device. Apple devices still potentially have security weaknesses which the FBI is asking Apple to exploit for them. Apple wants to fix these weaknesses, to stop Apple being forced to exploit them.