If a flight is F1 Y0 there should be no issue grabbing that for the OP. Yes, the flight may be overbooked but I have never heard of or experienced being denied the seat in this situation. I would assume it was a "there one minute, gone the next situation", which is completely plausible. I have clicked on a flight on AA.com but in the next step been told "sorry that flight is no longer available". I don't know why the agent could not simply have said that, instead of the "marketing issue" garbage.

This assumes that the OP had an F ticket to begin with. If you're ticketed in Y there is no way AA will stick you in F just to accommodate you. That sometimes happened before, when agents had more discretion, but those days are long gone.