Newsgroups and mailing lists are actually treated separately by Google. The user interface is identical, but the groups created within GG are not accessible through NNTP.
There's http://www.gmane.org/, which can create an NNTP-accessible archive of any mailing list. If that was set up early in the group's life then that's a good option. Alternatively whoever created the group might have the whole history in IMAP somewhere.
Once you start digging the whole story of Google Groups is so sad. Google bought up the few companies with a big historical usenet archive. That data should have been on archive.org. You can't properly browse those historical posts, only type stuff in a search box and hope you get lucky. The whole product hasn't been meaningfully changed in years, making you wonder if it's the next thing on the chopping block. It's textbook embrace, extend, extinguish.