Wow planckepoch. That's some bad luck! Unfortunately, one of the things we see (via this forum) is that many carriers and airports keep up the 'on time' until it is long past 'time'. There are complex reasons why they do this.
One of them can be that the aircraft to operate your flight might not have been the inbound from SEA. It could have been another inbound route, or a machine coming out from an overnight maintenance check. So jus tlooking at the paired flight number is not always reliable.
I agree that you are likely to get your money back and wait a few days before shouting further.
All of that said, I think the pilot did AA a big disservice. He could have said, "I'm sorry but, I'm flight crew. I can see a member of our ground staff over at desk #."