User blog comment:EmpyreanSmoke/Weaknesses/@comment-27036944-20151002015940/@comment-3045936-20151124031937

Castiel didn't say which order of Angels can or cannot be killed by Leviathans. He went to check on them, as the silence was bad for him. Because he knew Leviathans can kill Angels and stated Angels. It's a collective term UNTIL prove different.