If a new denomination of Jews require both parents to be Jewish for you to be considered universally Jewish, would that be considered a new branch of Judaism?

They either require all four of your grandparents to be Jewish (bilineal descent) or you would have to go through a c0nversion with their branch to be considered Jewish to them. Would that denomination still be considered a part of rabbinical Judaism because they fulfill the matrilineal requirement and they consider the rest of Torah/rabbinate law to be binding?

What would happen if that denomination begins to exist?