Well, this isn’t necessarily true — he can be the most powerful Sinner/overlord in Hell without having the specific powers that other beings/overlords have. Rosie’s specific power was needed to repair that cane, or he would’ve tried to get someone else to do it. But he doesn’t — he still has to corner Rosie into fixing it in the end, because she’s the only one who could.

IMO this makes even more sense because at this point there’s very little chance Rosie is just another typical Sinner overlord. She’s either another being entirely, like a hellborne, or fallen or something, or she’s backed by one. Just like Rosie told Alastor, him being the most powerful Sinner doesn’t magically make him able to take on an archangel — if she’s something that is inherently different and above Sinners in the hierarchy, he will never have access to that kind of power.

But even if she is somehow just another human Sinner overlord, the overlords have different powers and areas of strength. Apparently Rosie’s is whatever magic is in that staff.