Unfortunately, my free time for today got consumed by work and converting my own system to systemd-boot and writing a tutorial on how to add an entry for Windows on another ESP, which can be found here
However, while now looking into the stuff on the system I am familiar with and also confirming that the @saved entry stays functional when
a) recreating the .conf files through changing boot parameters and running sudo reinstall-kernels and also
b) adding and removing kernels, or
c) the update of the LTS kernel that landed yesterday in testing - which changed the displayed order
I think I can now say for sure, that @saved does indeed save text strings in the background, and not some hash or boot order number.
However, the .conf files for the kernels contain
sort-key endeavouros-6.1.12-arch1-1
And it could be, that @saved actually saves that string and not the string which is the filename of the .conf file.
However, it correctly saves the Windows-entry, too. Which could mean that it does still save the filename because the Windows-entry does not have a sort-key. And it can’t be the title, because that would match on all EndeavourOS entries: title EndeavourOS
I hope my free time tomorrow is enough for further investigations in that direction.