His comments are on current news stories, not how Barr should perform his job. Barr seems to be worried more about how he's perceived by everyone else, than he is about what the President says on Twitter. If anyone sounds like a baby, it's Barr who is crying that his job is hard enough without the President commenting on the news of the day.
Barr wasn't personally defensive in these remarks. He simply pointed out that one particular type of public communication -- specifically, about active DOJ cases -- undermines the DOJ.
When someone in a prosecutorial post says this, what he's saying is that this type of public commentary makes it harder to prosecute cases.
I think Barr just dropped a big hint about what's going on in the DOJ ... and he didn't have the Roger Stone case in mind when he said this.
He needs to explain how it "undermines" him doing his job, and how it "undermines" the people working under his supervision doing their jobs. Obviously comments about the Roger Stone case from other people on Twitter, etc., or from the liberal media don't "undermine" him or the total DOJ...just the President's comments.