That dino wasn't very reactive. The stone throw should really have been enough to majorly startle the animal and make it reconsider being where it was. Instead it just stood there, no snarling to intimidate either, and basically did not react to having a staff pointed at it and the fireball just thrown. Maybe the casting was very fast, but I'd generally expect an animal which hunts to have good reflexes.
As for the second dino, it apparently wasn't any faster than them since it hadn't caught up by the time they got back to the village.