It means that you can install sysvinit or any sysvinit-compatible init, and all daemon packages still provide init scripts, as painful as that is. And Debian still supports installing systemd-shim instead of systemd, so you can have a desktop system with a non-systemd init and it'll still function.
> In practice it's not possible because of so many (unecessary) dependencies on systemd.
Such as? As far as I can tell, almost nothing depends on systemd. A handful of things depend on libpam-systemd (for session management), which functions with systemd-shim.