That means createrepo_c has not been compiled with DeltaRPM support. Where did your createrepo_c package come from?

c.f.: https://www.mankier.com/8/createrepo_c#Options---deltas

createrepo_c-0.10.0-6.el7.x86_64, which is confusing. According to https://github.com/rpm-software-management/createrepo_c/releases this would appear to be the latest release.

Looks like drpm-devel isn't a build dependency here: https://koji.fedoraproject.org/koji/rpminfo?rpmID=9140306

For comparison: https://koji.fedoraproject.org/koji/rpminfo?rpmID=12366387

Without libdrpm linked in, the DeltaRPM logic is disabled.

It's probably worth filing a bug against createrepo_c in EL7 to request this to be fixed.

I was going to give that a quick install to verify that resolves the issue, but I don't see any such package as libdrpm or drpm-devel there. In fact:

$ yum search all drpm
Loaded plugins: fastestmirror, versionlock
Loading mirror speeds from cached hostfile
================================= Matched: drpm ==================================
drpmsync.x86_64 : Sync a file tree with deltarpms

It's probably worth filing a bug against createrepo_c in EL7 to request this to be fixed.

Filed at https://bugzilla.redhat.com/show_bug.cgi?id=1538650

I would close this one, as it should be solved outside of koji. If you feel, that there is something koji can do about it, please reopen.

This issue has been migrated to Fedora Forge:
https://forge.fedoraproject.org/koji/koji/issues/790

Please continue any further discussion there.