Yeah, that person either doesn't entirely know what they're doing or just had a brain fart moment. They put 0x999 when writing the code, which actually gives 2457 rupees. The game still caps rupees at 999, so it may only display 999 even though the value in RAM is forced higher.
Still at 0 titles that do what you describe.
Those codes do indicate that you need a pointer for rupees, which is why the other person is having so much trouble finding the address (the value moves to a different location in memory in certain circumstances, as opposed to using a static address that never moves relative to the start of the game's memory)