I think it’s someone else.
It could be a way to tie in Monsters obviously not taking place on Wano (clothes, names, architecture are nothing like Wano).
But it would open up a lot of weird things, like Ryuma then killing two seperate dragons (one over Wano, one in Monsters) and some strange Shimotsuki family history (did Ryuma marry into them? Was there another branch that had left Wano previously and went back? Did they change their name?)