OK, I'll explain it a bit more verbose.
For all of download.o.o, we have
MirrorBrainExcludeFileMask "(/[^0-9]*|\.(repo|ymp|json))$"
which means to serve all *.repo, *.yml and *.json files that don't have digits in their filename directly from download.o.o. This also means that all other files get redirected to a mirror. This typically means RPMs which have the version and build number in their name. These files never change. They get deleted and replaced with new RPMs that have a different filename (new version or build number).
Now have a look at the files in .../repo/oss/boot/ - the files there don't have a version number in their name ("yast2-trans-xy.rpm"). Mirrorbrain scans and redirects only based on the filename (checking the content of all mirrored files would be insane) and therefore only knows if a mirror has yast2-trans-xy.rpm, but doesn't know if it has the latest version or an outdated one.
With my change, alll files in .../repo/oss/boot/ get always served directly from download.o.o instead of redirecting to a mirror. This adds a bit load to download.o.o, but prevents that users get redirected to outdated mirrors.
From a mirrorbrain perspective, the perfect solution would be to have version numbers in the filenames ("yast2-trans-xy-20190614.1.rpm") like for all other packages, but that's probably much more difficult than disabling mirrors for this small set of files.