1. Attending a military academy doesn't mean you are gifted with boundless knowledge and intellect or that you stayed awake in every class.
2. Unless a member of the military is participating in a mission that affects the civil population (civil unrest/ natural disaster/border defense, etc...) they don't get briefed on Posse Comitatus as a constraint to their mission.
It could be that the officer in question just never had those experiences and the first time he heard the term was from your friend.
Also, with all the recent "purges" of old school senior military officers in all branches, a steady flow of career-climbing, PC-infused successors have risen up to impose the Left's idea of military service onto the ranks. It's one of the reasons my buddy is hang it up at the end of the year---he can no longer stand the latest incessant PC indoctrination that is changing the mission from warfighting to liberal social engineering. He's just had enough of it.