Dear Avira,
Maybe they did treat some or all of them like tinokos shenishbu. I don’t think they were tinokos shenishbu at all. But when the Ramban says that most of the people forgot everything, he was saying what he knew to be true.
And I don’t know where to find about Bayis rishon in that book. As far as I know Josephus skips the malchai yisroel there. Try Antiquities. But if that’s all you have, see his descreption of the Jews in the Diaspora.