It's F1 but there are three seats open. It's very well possible that there is more than 1 F seat available. AA is just allowing one seat to be sold, possibly holding one or two back for the oversell situation.

Upgrades will be done before standbys, so that can open up seats. And hope for no shows. I don't know how elite-heavy these flights are on a Wednesday afternoon, but if Y is full and F is open and all upgrades are accounted for, a GA can clear you directly into F (clear standby+upgrade) for stickers, though this is not policy.