Comment # 4 on bug 1021969 from Tony Mechelynck

After running zypper up for that patch, it advises to run "zypper ps -s"
because some processes are using deleted files. Following this advice shows
that the systemd (and systemd-journald) executables currently running are
deleted.

AFAIK, the only case where updating systemd would not require a reboot in order
to execute the newly installed systemd executable is if, at that moment, the
system were running under Sys V init instead; however, the same patch includes
an update to systemd-sysvinit too, so a reboot would also be needed then, in
order to execute the newly installed sysvinit.

You might call this a question of principle: without rebooting, the newly
installed systemd (or sysvinit) executable would never be run AFAIK.