There is a large group of languages which are closely related, including Assamese, Bengali, Bishnupriya, Manipuri, Chakma, Chittagonian, Hajong, Rangpuri, Kamtapuri, Rajbongshi, Noakhalian, Rohingya, Surjapuri, Sylheti, Tanchangya - all spoken in that part of the world. To assert that the Rohingya are Bengali just because they speak a language in the same family is ridiculous. It's as ridiculous as suggesting that Spaniards are really Italian because their languages are similar, or that the English are really German because their language evolved from North Sea Germanic (Ingvaeonic).