i think it is more expanding his lore due to the fan recepetion and less about retcon.
They said mueh'zala was the loa of dead to all trolls, but when he raised bwosandi the other tribes prefer him, and only after a time they all forgot mueh'zala except the sandfury.
Since it take a heck of time to forget something like this, Bwosandi ascension is either by the time the zandalari empire was being formed or before that, so he could not be from any of the other trolls subraces.