The real difficulty, or rather the real subtlety, is not nesting but name binding and the automatic renaming of bound variables where necessary.
If you're trying to understand what a closure does, perhaps that's true.
If you're trying to understand how it does it, and you come from an imperative programming background where you know the ABIs for your languages like the back of your hand and are used to the kind of function set-up that always allocates space for local variables on the stack, there is a huge question of how all that lovely, neat theory is actually implemented in practice.
Some of this discussion reminds me of the jokes about professors who prove that a solution to some elegantly expressed problem must exist, but then lead it to the grad students to actually find it...
If you're trying to understand what a closure does, perhaps that's true.
If you're trying to understand how it does it, and you come from an imperative programming background where you know the ABIs for your languages like the back of your hand and are used to the kind of function set-up that always allocates space for local variables on the stack, there is a huge question of how all that lovely, neat theory is actually implemented in practice.
Some of this discussion reminds me of the jokes about professors who prove that a solution to some elegantly expressed problem must exist, but then lead it to the grad students to actually find it...