The rule in American law that a sovereign cannot be defamed is distinct from any question of whether a group can be defamed, and that’s the key question in this case, since federally recognized Indian tribes do hold themselves out as sovereigns.

I also think Beauharnais is no longer good law, but there’s little reason to think this case will reach that issue.

Because defamation of a group is not actionable? That’s not clear, is it? If I’m not mistaken the governing case is Beauharnais v. Illinois (1952), which allowed prosecution for a group libel. The Supreme Court has tended since to give more weight to freedom of speech but has not actually over-ruled Beauharnais.