Right. Because of their nature, the angels could only chose once. Their choice is permanent and cannot be repented of or changed. That is because, ss I understand it, they see the essence of goodness and neither act nor will agains it unlike we, who can chose the good and later chose evil because, well, Original Sin.... Up until the moment of death we can choose good or evil. That truth demolishes the idea of onced saved always saved
Angelic rebellion is more serious because they are not tempted by flesh. Their rejection of God is purely spiritual in nature and therefore cannot be excused.