We still don't know who used these boats right?

I'm at ch21. The Imass who got their bodies back have reconciled with each other.

I'm talking about the canoes that Bivatt saw at the northern shores of lether carrying around 500k people. I initially thought they were just grey swords but we only have like a few thousand grey swords so that can't be it. Is this another RAFO or I missed something?