And OSS projet based on the SFOS core exist : https://nemomobile.net/, https://github.com/nemomobile-ux
If we're going to start counting forks, we get to count LineageOS and GrapheneOS for Android, and then the goalposts really move.
The main point is that AOSP as a system (modulo firmware) is open source and SailfishOS is not. Also, even though Sailfish has an Android compatibility layer (though only for official devices), compatibility is most likely always going to be worse than 'real' Android.
That said, I hope that Jolla Phone becomes a success, more competition is good. Hopefully being funded better will move them to fully open source the base system.
A true AOPS image is missing most core Apps.