The phrasing is slightly ambiguous, but I don't think the author is saying that Rhea Silvia's son was Romulus as a fact. I think the author is using the fact t

The phrasing is slightly ambiguous, but I don't think the author is saying that Rhea Silvia's son was Romulus as a fact. I think the author is using the fact that the Roman's had a legend/myth that their founder who was the son of a Vestal priestess (even amongst other, perhaps mutually contradictory legends) as evidence that the Roman's held the Vestal priesthood in high regard. This would be akin to the whole Washington cutting down the tree thing. It's a myth, but you could also use it as evidence that Americans for centuries admired honesty in their leaders.