Where did the thread say they might have or don't have a car for confirmed reservations?
It's true they were sold out when the OP arrived, but her main peeve was that they did not inform her and she stood around wasting time for their shuttle to arrive, when she could have been gotten a car at a different agency and been on her way.
A lot of stations at each car rental company probably sells out at any given time during the year for a variety of reasons, that's not a National specific or Key West problem. Lets face it, the goal of a rental agency is to rent every car out. It's how you handle new incoming customers when you do sell out that counts. As a EE with National, she expected and should have received better communication about the situation at hand.