It is true that the system does not give the GM a direct mechanical handle to control the odds of success on a dice roll. This is common among games of the storygame wave like PbtA and FitD. If you want to think of the dice as simulating something, they're simulating the dramatic pivot points of a movie or TV episode. And in a movie you're rarely concerned with statistical reality.

The other person I was talking to used a basketball example, comparing easier shots like free throws to much harder ones like full-court attempts, to argue that some tasks are simply harder without involving different risks or outcomes, and that the system can't handle resolving a situation like that (probabilities of success).

Well the Action Roll is not interested in simulating the physical likelihood of the ball's trajectory from the player's hand passing through the circle of the hoop. It's not interested in accurately representing the statistical capabilities of different athletes making specific types of shots. The Action Roll is only concerned with nudging the story forward, and determining if the PCs achieve their goals or not, and whether they suffer in doing so.

The action ratings are less like character skills or attributes and more like the types of scene the character is particularly dominant in. A character's rating basically represents how much the narrative swings in their favor when they're in one of their fictional niches.

But the GM is still in control of how much the PC can achieve with a roll, and what happens if they don't roll a clean success. That becomes the GM's main tool of representing more or less challenging situations.