Now we're in 2.3, we have the answer to that question.

Mistbeard isn't only one person, but many : they just wear the same helm/mask.
Eynzahr kind of confirm he was Mistbeard :
But now that legend will come to an end, for the last Mistbeard has found a new purpose in life: one woman and her dream of a brighter future for Limsa Lominsa.


Make sure to read the quest when you finish the Hullbreaker dungeon