Yeah, I agree. While this is a terrible move IMHO, from my superficial reading of the GPLv2 it doesn't really constitute a violation: the license imposes that the source be distributed to anyone who asks, potentially even charge a fee to cover its distribution costs, but it doesn't require that development happen in the open.

My guess is that all their code is in their monorepo (google3), and they don't want to set up a tool to sync it to a public git repo, so the easiest way is just to have someone create a tarball on demand.