Instead of completing the whole line, which doesn't work, only complete
the pid, but still show the whole line so the user can see which command
was which.

Users can also let the parameter expansion sort the completion by date
instead of by pid, by setting

zstyle ':completion:*:*:systemd-coredumpctl:*' sort no

so that the zshcompsys doesn't sort the _describe function for only
systemd-coredumpctl.