Although exploded srpm is the normal CentOS SIG repo format, there are some that use the 'flat' dist-git style repo format. When this is the case, centpkg-sig {sources,srpm} do not work.

Example:
centpkg-sig clone rpm
cd rpm
centpkg-sig switch-branch c9s-sig-hyperscale-experimental
centpkg-sig sources
centpkg-sig srpm

Both the sources and srpm command fail with

Downloading rpm-4.16.1.3.tar.bz2
######################################################################## 100.0%
Remove downloaded invalid file /tmp/tmp/test/rpm/rpm-4.16.1.3.tar.bz2
Could not execute sources: Server returned status code 404