Whether Rohingyas came to Myanmar 600 or 5000 years ago is meaningless.
When Myanmar was founded in 1948, you did not deny the citizenship of Rohingya people. When Myanmar established Rakhine state in 1974, you did not deny the legal residence right of Rohingya people. Therefore, Rohingyas are Burmese from any point of view.
I can't confirm the origin of Rohingya people, and Chinese data also show that there are two statements at the same time, maybe only the British know the real situation.
However, in any case, the current Myanmar government has no right to deny the rights of Rohingya people.
If you want to deny the origin of Rohingya people from the Pagan Dynasty, it also means that you do not recognize all Myanmar dynasties after the Pagan Dynasty. Are you going to return all non bagan territories? They also joined Myanmar after 1400s.