@zbyszek, Well you've asked if the update was marked as security and the answer is "yes it was" fedpkg update template doesn't have severity field in default template nor in one suggested by security team https://bugzilla.redhat.com/show_bug.cgi?id=1550246#c1 and not everybody uses bodhi UI. I hope it answers your questions. Even more all information available at the right side of this page (FEDORA-2018-5051dbd15e ) and it says :
Update Type security Update Severity urgent
So I don't understand why you keep asking for information you can find yourself very easily.
As it was said ndiff was not usable if frontend is not not installed. In case if ndiff worked before the upgrade (nmap-frontend installed) it will work afterwards because frontend depends on ndiff if not.. it means user didn't use it.