In fact, it doesn't even fit the history of the Clayne- the only major reference I could find on the wiki talks about them in the context of ancient Tevinter, not a force in the fall of the Dales. Perhaps Illan was refering to a more recent movement?
Tribes are weird. Even if the organization doesn't survive for very long under the same name, the
name can survive for a long time attached to other groups. There were Suebi/Sueves from the third century BC to the seventh century AD, for example, and it rather strains credulity to imagine that a single tribe with a single culture managed to persist for that long.
Alternatively, the name "Clayne" could be a referent employed solely by outsiders. These sorts of names were often blanket terms. The Romans were especially notorious for using them. They called everybody north of the Walls in Britain 'painted ones', or
picti. They called everybody northeast of the Rhine/Danube line
germani. They called everybody from the Eurasian steppe
scythae. None of these overarching terms would have meant anything to the people that they actually described; there was no inner unity among the various tribes in the
Germaniae, for example. So "Clayne" might simply be a blanket term for a bunch of south Thedosian barbarians.
Or Ilen could just have been classicizing.