The Albanian video is a fairly extreme example of the sort of vocal heterophony that emerges when people sing together; as Poppygator pointed out, people are incapable of singing in unison! Some would call this polyphony, but when one thinks of polyphony one invariable thinks of orchestration, and this is so much more immediate than that somehow. This isn't to say these Guy's don't rehearse, on the contrary, just that it comes out of something living rather than written down, or else composed.

Doo-wop emerged on the streets of Chicago in much the same way, and there's no reason to believe that it was any different with sea-shanties or chorus songs in the English tradition. We hear it in the singing of The Coppers, The Watersons, The Wilsons and we heard it in The Young Tradition too; it's certainly there in Shape-Note Singing and other English speaking traditions, so why wouldn't it be there in the carolling of yore?