h/t @arrfab who kindly pointed me to https://sigs.centos.org/guide/delivery/#centos-stream-9-and-above_1
It turns out we do already have metalinks for our source repos, but the repo id we provided is wrong.
(Re-)discovered this issue (see https://pagure.io/centos-infra/issue/978) when adding Hyperscale repos to rpmdistro-repoquery, see https://pagure.io/rpmdistro-repoquery/pull-request/8
Need to now fix centos-release-hyperscale as well as mock configs, where even the binary repos are not using metalink yet
centos-release-hyperscale built and tagged https://cbs.centos.org/koji/buildinfo?buildID=42459
PR for mock configs https://pagure.io/centos-sig-hyperscale/mock-centos-sig-configs/pull-request/6
Looks like this is done, closing.
Metadata Update from @dcavalca: - Issue status updated to: Closed (was: Open)