Is it possible that substance X arrests cells in additional points that are
not observed in this experiment?
This is unlikely. Since the experiment was performed on a non synchronized culture and lasted the duration of a full cell cycle of these cells, all cells could reach the arrest point/s.
Would using a synchronized culture change your decision?
Submit a well justified but short answer!!!

This possibility is more likely in experiments using synchronized cultures, where the arrest point that is closer to the synchronization point will be observed, provided the arrest effect is strong enough.