Package names are a bit confusing here. The systemd package includes all the binaries - one of which, logind, is required by GNOME - but doesn't install systemd as /sbin/init and doesn't boot with it by default. The package to do that is called systemd-sysv, for reasons having to do with the special treatment of sysvinit as a "required" package in dpkg.

So 7.5% of Debian systems have it installed, but only about 0.3% (plus however many of that 7.5% that are setting init=systemd in GRUB) are using it.