Was Volsung Fa actually Ragnar and Sefi’s father?

I finished LB and loved the reveal about Volsung Fa, but now I’m confused: is he the true father of Ragnar who had since become an agent of Atlas, or is he entirely an imposter pretending to be Vagnar Hefga?

Was this ever directly revealed to us? I concluded that he’s an imposter, but I’m not sure if that was ever revealed in the story.