On paper, this new development seems like a really egregious retcon.
After the Cataclysm devastated their island, the Prophet Zul banded together the scattered troll tribes into one empire and have been trying to conquer Azeroth. Vol'jin and his Darkspear were the first to rebel against this and assisted both the Horde AND the Alliance to quell their uprising in the rebuilt Zul'Aman and Zul'Gurub.
The Zandalari returned again on Pandaria where they helped resurrect Lei Shen. That too took the combined efforts of Jaina Proudmoore and Lor'themar Theron's forces to stop them as well as the Mogu empire.
Now you expect me to believe that they will be willing to be chummy with the Horde after their combined forces killed so many of their kin? And where is the Prophet Zul while this is happening? Is this a different faction of Zandalari while Zul is leading a band of renegades elsewhere that need to be dealt with?
So many questions and I'm not finding any answers.