Sorry but this doesn't confirms anything and is all headcannon.
Sure to have Pluton buried with old Wano means that Pluton was already there when the borders were made. But that doesn't make that all that only happened after Void Century.
Pluton as Ancient Weapon pretty much was the reason why Wano didn't even needed to close themselves. Many things left explanation to come with such vague conclusions.
No, because Pluton's origins is traced back to Water 7, where it was created. The only logical conclusion is that after it participated in the war, it was brought to Wano and sealed once the war was over.
You wanna talk about headcannon, when your disregarding whats logical, and making things up like "Pluton is the reason why Wano didn't need to close themselves" when the ships origins aren't even from Wano.
Then, there is also the simple fact that Joyboy is linked to Wano through Zunesha, and yet the people of Wano have never heard that name until Luffy defeated Kaido, and think Luffy is Joyboy. That's another indication that Wano's history during the Void Century has been erased. The people can't know about Ryuma who lived during the Void Century, but not Joyboy, who is heavily linked to the Country through Zunesha and the Kozuki Clan.
Wano citizens don't know about Zunesha. They don't know about Joyboy, but they do know about Ryuma. That's proof that Ryuma did not live during the Void Century, otherwise all his exploits would have been erased as well.
The third and final nail in the coffin, is when Sukiyaki spoke about the Wano of 800 years ago, he straight up said that what happened back then in the past, is unclear to him. That means, that even the Clan who has lead the Country for the past 800 years, doesn't know what happened during the Void Century, or why the walls were constructed around Wano.
Everything that happened during the Void Century has been erased from One Piece history. Ryuma is not some sort of exception to this.