Ok I suspected that would be the new effective way and I updated the script (generating the debian gnome status pages) to use the gnome-build-meta tarballs, in a manner similar to yours (from a quick look).
However it would help if the buildstream tarballs were hosted in a predictable location : this announcement points to /sources/gnome-build-meta/47/gnome-build-meta-47.3.tar.xz while the 46.8 announcement has /teams/releng/46.8/gnome-46.8.tar.xz, and 48.alpha has /sources/gnome-build-meta/48/gnome-build-meta-48.alpha.8.tar.xz (an extra .8 to the version number)
The extra .8 to the alpha release is an artifact of the multiple tries to use the release service. It should be the only release having this.
The 46.8 release was manually moved to be consistent with older tarball location, whereas the 47.3 and 48.alpha weren’t. The most likely situation is that we’ll start using the new location (sources/gnome-build-meta) going forward, and keep using the old location for the remaining 46.x release. You can use the cache.json file to discover the latest version.