I don't think this is true at all? AOSP is completely open source modulo driver blobs (which Sailfish has too) and Google services.
One can make a fully functional system, modulo drivers, out of only open-source components using AOSP. It's not possible to do this using Sailfish; the compositor, UI libraries (Silica), and most of the "core" apps are still closed source.
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.
A true AOPS image is missing most core Apps.
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.