He was a Keynesian in some ways, but an Institutionalist at heart.
So when he makes these criticisms he's coming from a theoretical background that is in a lot of ways more similar to Hayek than to Keynes and the monetarists. He is looking at trends in corporate power and behavior as the explanation for these issues. And he is looking at the Keynesians as people who are talking about different stuff.
So my guess for how this came about is that he's not an insider chastising his fellow Keynesians for naiveté. He's an outsider in a dying theoretical tradition taking a potshot. And the RESTAT editors rightfully called B.S. (or he realized it was B.S. before it got too far along).