I've seen this a few times- GDS availability goes all bonkers just before a flight and imho isn't a reflection of loading once 'control' moves to the airport (T-4hrs?).

I've got the last J seat (while standing by on a coach ticket for a flight showing J9Y0) an hour before the flight.

I've also got a birthday upgrade (J->F) on a flight that was F0 on the last few days leading right up to the flight time.