My speculation is after wathcing that video, were Guldan are trying to summon a portal. We don`t know for what or for who was that portal, but I quess, that portal was for Sargeras, but Guldan faild to summon that portal. So Sargeras can`t be a finale boss in Legion.
https://www.youtube.com/watch?v=WI-WApMhXlM&t=1s