With the exception of the enemy commander in the special stage, the enemy girls are units we don't have yet which are obtained through Exclusive gacha (then Platinum afterwards), so it's just teasing, yes. They're not event units. It's not the first time they've done it, though. During the Christmas event Xmas Lisette was an enemy unit during an event stage but she wasn't available like she should have been. New Years Pas was the same way.