It was rude simply take the seat with luggage in front of it, simply because the other person had gone to the restroom, the comment "Here at American Airlines, we don't tolerate hostility of any kind" sounds like a threat to potentially stop the OP and spouse from travel.
I would have immediately requested management for them to know who she was, and would have expected them to report on the incident, as I would have certainly written to AA about it. She was the one a bit out of line to take the seat, and way out of line to make that kind of comment, and I doubt it was the first time. These kind of people do NOT belong in this business.