I'm not sure it's using tag annotations; I'm not seeing evidence of them. Plus, if I clone a repo with releases, then push (with tags) to a new repo, the new repo doesn't have any of the release notes.
Weird. I'm seeing some evidence of my tag annotations, but the first parts are cut off and I don't see a way to view all of them. Maybe they are still getting the kinks worked out...
Hmm. I created a dummy package and allowed GitHub to create the tag for the release. It looks like it used a lightweight tag, and I don't see any evidence of any extra refs:
$ git ls-remote https://github.com/dlitz/dummy
23ed6e532fb4a3eb1403e0091885ccdb9f09b577 HEAD
23ed6e532fb4a3eb1403e0091885ccdb9f09b577 refs/heads/master
23ed6e532fb4a3eb1403e0091885ccdb9f09b577 refs/tags/v0.0
Pull requests, on the other hand, are stored as refs in the remote git repo: