Thenewwwguy
Deity
i think the implication made is that they belonged to an ethnic group that no longer exists but was assimilated into the vietnamese ethnicity, but like you noted, at the time of Seondeok’s rule, Koreanism as a unified ideology or ethnicity didn’t exist, Sillan was its own language, own culture, own ethnicity. Specifically, the Trung Sisters were Baiyue Chinese, an ethnicity that was incorporated and absorbed into Han Chinese over time. In that aspect, yeah, they weren’t vietnamese at the time, but Vietnam as a political concept didn’t exist at that time either. Southern ‘modern’ vietnam was the Champa empire, for example. So I think in that sense, making them leaders of Vietnam would be partially inaccurate but acceptable.Why are they not considered Vietnamese? Do you mean ethnically they aren't Vietnamese or is it the fact that they were born into Chinese occupation and the actual state called Vietnam didn't appear until later?
Even if that's the case that didn't stop Seondeok from appearing and leading a unified Korea.
Interestingly enough, Trung Nhi (the younger, non-ruler sister) trained an all-women militia to fight back against the chinese. It would be a perfect UU, or an LUU if Trung Trac is the leader