Hmm, not sure that only Zunisha can open the border. "Open the border" was also always what Oden kept repeating, and iirc Oden wasn't shown met or interacted with Zunisha.
So if Oden could open the border too then Zunisha isn't required.
Do you also think Kaido knows Pluton is hidden in Wano (thus he kept trying to occupy Wano) but didn't have it in his hands yet? What was Kaido's intention again by moving Onigashima to Wano main island?
P.s.
@Echizen_Jo_Ndule said Pluton might have been hidden in Mountain Fuji. Probably that is true. We kept hearing Mt. Fuji yet Oda never focused on the mountain yet. It is as if Oda teased the mountain is special, but even until Kaido's defeat he still saves the mountain secret. Maybe Kaido has searched all over Wano from land to village to ruin to below the sea but never thought that it is actually hidden inside the mountain. (Although it is weird too because why dragon creature like Kaido avoids the mountain?)