> Cargo-cult HOWTOs is the only explanation I can think of.

Pretty much. When I google "raspberry Pi time machine," every result I see, except one, tells you to do the HFS+/AFP method. These results include content churning sites like techradar and howtogeek.

The only site that recommends ext4/smb that comes up, fourth on my list of results, is this one: https://mudge.name/2019/11/12/using-a-raspberry-pi-for-time-...

netatalk is also the wrong solution nowadays. AFP is unmaintained, both on the netatalk side and on macOS. Instead, Samba supports Time Machine backups via the "fruit" (!) extension, which also provides some nice performance improvements for macOS clients.

https://manpages.debian.org/buster/samba-vfs-modules/vfs_fru...

TL;DR though: add the following configuration to the global section of your smb.conf:

    vfs objects = fruit streams_xattr
    fruit:aapl = yes
    fruit:metadata = netatalk

And under the section for your Time Machine share:

    fruit:time machine = yes

SMB on ZFS also works excellently, 4 years and counting with many power losses, ungraceful ejections, etc. never a corrupted backup.