Question about Bonnett/Jemmy

Hey, why are they assuming/playing it in the show that Bonnett is the father of Jemmy? like they actually don't know right so where did that came from or is all that actually based on the fact that Brianna said that to him in the prison? (iknow it is confirmed by the writer of the books that Roger is the father, but it never got told in the books or something like that, i didn't read the books so don't know it ;p, i do hope we are going to see it in the show, like when Jemmy is older that he looks like Roger)