Kohen Kusi

The gemara explains the mishna on 56b that we believe a kohen with regard to where a grave is only if it's a Kohen kusi eating truma there. How could there be Kohen Kusi if the kusim were geirim? I think that the simple answer is that the Kusim started off as geirim but like any group/cult there are always converts later who think that their way makes sense. So it's possible that there were some Kusi Kohanim. The Artscroll though quotes a Tosafos Yom Tov who explains this historically:

The Gemara here refers to the descendants of the Kohen brought by the Assyrian king to teach the Cutheans "the law of the god of the Land' (see II Kings 17:27). Apparently, the unfortunate results of this was that his descendants ultimately adapted the ways of the Cutheans.