As far as I can tell, the only benefit of this scheme over content hashes and signed outputs is that Guix can serve their updates over HTTP. Otherwise it's vuln

As far as I can tell, the only benefit of this scheme over content hashes and signed outputs is that Guix can serve their updates over HTTP. Otherwise it's vulnerable to the exact same scenarios as Nix: the rogue Git forge employee/maintainer can replace signatures just as easily as they can package sources.