I note that a tribe is considered wiped out once it no longer holds any islands. This is usually accurate, but, in theory, if it still has any ships in transit, it could still conquer an island and come back.
I just saw this actually happen. I was playing Rus. I left Nor barely alive - one island with population 1 - and focused on taking down Gothi. Just before I conquered the last Gothi island, it emitted a flotilla aimed at that last Nor island. I conquered the Gothi island with the Gothi flotilla still in transit and was told I had wiped out the Gothi. Out of curiosity, I let the ship arrive and conquer the Nor island (I assume I could have forestalled it by conquering the Nor island myself and winning). I then conquered that now-Gothi island and was told, again, that Gothi had been wiped out (and got the Victory screen). Apparently it doesn't break anything, but it looks a bit odd.