I agree with you. As of now, the Xebec is alive theory is only head canon, but the thing with people not sleeping in snowy countries, because they would die otherwise, was the only other time stuff like this was mentioned, so naturally people would take this as a connecting point.
The interesting part is, who were the people that told the stories and from where did they hear them?
For one, we had Luffy overhearing a conversation in a "tavern".
I would like know what tavern he meant.
And later we heard a similar thing from Buggy. He also heard it from other people, but who?
It could be some sort of legend. As you said, people hear stories from time to time. Oda might even retcon this into never existing at all. Who knows? It could be from his lineage or maybe from his Mothers' side, dating back to the Void Century or something.
But interesting that you mentioned the Void Century. Maybe this was also some sort of punishment / curse on the Blackbeards family, similar to Zunesha being punished for walking for all eternity, because of a committed crime. It would be interesting to know. Does Zunesha ever stops to sleep?