I can see two scenarios in which Mueller would speak truths beyond the public parts of the report.
1. Barr really p-ssed him off. Not only did Barr willfully and cynically misrepresent the report, he also lied about his interactions with Mueller re: it. Mueller's honor code may have been activated by this.
2. The GOP loons go after him at the hearing to grovel to Sh-tler and he says, "welp, if that's how you want to play..."
Mueller seems to be a straight shooter but he is at the end of the day a Republican so he likely doesn't want to see the party reduced to a smoking crater. If they and Dump aren't too outrageously stupid he will probably pull the covers over his head and pretend there's nothing he can do to stop the Nazi Night Train. But if they impugn his team that "leave no man behind" machismo may kick in. If I were the Goopers I wouldn't even show up to the hearing for fear that some backbencher says something Fox News level stupid and untrue and Mueller opens up a can of whupas-s on him.