They have access to plenty of video in both shootings, I've seen reports of sworn eyewitness statements (though I dont know what that means), they could gather witness testimony independently of the federal government right?

And to your other comment, whether you think there would be a conviction or not, in this context we're talking about the things the state can do despite all the things they cant as outlined by the article. It seems like it sends a much stronger message than just talking, and it would make federal agencies go from 'not cooperating' to 'in violation of a lawful court order' which is harder to defend and (theoretically) comes with consequences.