Okay yes, I just killed 4 in 2 boat rides. They definitely drop birdman's cape. Both times an apkallu jumped on board the ship. then 5ish mins later another apkallu jumped on board. Then a few minutes after that one depopped and became almighty apkallu. While fighitng him, the other one depopped and became almighty apkallu.

So by not being alone I think it means there needs to be more than 1 unclaimed apkallu on board.