frr-7.5.1.tar.gz downloaded from distfiles.gentoo.org (6746148 bytes) is different from official tarball downloaded from https://github.com/FRRouting/frr/releases/tag/frr-7.5.1 (5840772 bytes).
Tarball used is:
And it's still the same as what's on the mirror, it was just generated differently (so nothing strange going on).
But I guess the release tarball does have a pre-generated ./configure and is smaller (notably the .xz), so I guess it could be used to skip eautoreconf and setting S= (no frr-frr naming).
Unsure if a particular reason it's not used (maintainers should know).
They didn't produce one for the 8.4.2 release either.