Jean and Emma are clearly more friendly than they've ever been. Their relationship is snarky but they've gone from dragging one another for filth to playful ribbing. I understand preferring to see Rachel train Jeen, if only to give her a shred of narrative importance, but Emma was probably chosen in an attempt to redeem herself/atone for her role in the Dark Phoenix Saga and her antagonistic relationship with adult Jean, who was still dead at the time.