git:// is not just unauthenticated, but unencrypted on the wire. They want you to use https:// for unauthenticated clones to at least prevent MITMs.
Like most crimes, the biggest and most common dangers come from the people/software you already know. It's going to be a company's profit incentive or a lone dev's hissy fit expressed through softare you implicitly trust that gets you. Unencrypted internet transfers have worked and will continue to work for a very long time with very few problems for the vast majority of people. Requiring encryption might be okay, bu…
CA-based TLS is a requirement that a client can enforce on the servers, not viceversa (except for client certificates which are obviously not the case here).
When a server (Github in this case) chooses to acquire a CA-based TLS, the client isn't forced to depend on the CA in any way - it can even choose not to verify the certificate authenticity.
If the official git implementation started requiring TLS, thus forcing free private git servers (including your personal selfhosted gitea or whatever) to centralize under a recognized root CA, then your comment would make sense.