New Keynesians believe that Say's Law is false. But they want the central bank to try to make Say's Law look true.
New Keynesians believe that real business cycle theory is false. But they want the central bank to try to make real buiness cycle theory look true.
New Keynesians believe that the loanable funds theory is false. But they want the central bank to manipulate liquidity preference to try to make the loanable funds theory look true.
New Keynesians believe that the economy has zero self-equilibrating tendencies. But they want the central bank to make it look like the economy has powerful self-equilibrating tendencies.
New Keynesian model + New Keynesian central bank = "classical" economy + random error (because no central bank is perfect).
New Keynesian model + non New Keynesian central bank (like one that sets interest rates on a whim) = godawful explosive or implosive mess.
New Keynesians are engaged in a terrible secret conspiracy to try to make the economy look very different from how they think it really is. Except it's not terrible, and not secret.
"Why [is mainstream New Keynesian economics] brackish? Because it has retained the nonsense of marginal productivity distribution theory while discarding the foundations of Keynesian economics. The essence of Keynes’ economics was the liquidity preference theory of interest rates and rejection of the claim that price and nominal wage flexibility would ensure full employment. New Keynesians abandon both. They replace liquidity preference theory with loanable funds interest rate theory and they use price and nominal wage rigidity to explain cyclical unemployment."
No they don't. But they want the central bank to try to make it look like that.
(Which is not to say I think New Keynesian macro doesn't have big problems. But these aren't them.)