The daf Millhouse seems to be talking about is 16b in Yevamos. It is halachah not aggadeta; he’s right. It’s talking about whether or not a marriage between a goyish man and a Jewish woman might be a valid kiddushin because of the possibility that the man is from the 10 lost tribes. It gets shot down essentially because the 10 tribes are separated and therefore it follows mechtzah al mechtzah: rov goyim are not from the 10 tribes therefore we assume each individual goy is a goy gamor (if I’m understanding correctly; I just skimmed).
I have to agree with Millhouse that, based on this, it seems like anyone who tries to claim a group is one of the lost tribes would be arguing on this gemara. It’s a good question, then, why chushuve people have brought up the concept over the years. Is there a gemara somewhere else that shlugs this up?