I knew I shouldn't read this post by Simon Wren-Lewis but I did. I also refer to this post.
I fear I disagree. I haven't read Keynesian Economics and The Economics of Keynes, so I should just be silent. But I think that Leijonhufvud is a new Keynesian in the same way Prescott is. Nominal rigidities are important to New Keynesian analysis and Prescott absolutely rejects the idea that they are relevant to the study of the economy. Rational intertemporal maximization by a representative agent is central to new Keynesian analysis (recall you called it the heart of modern macro) and Leijonhufvud (in seminars and over dinner) absolutely rejects the idea that it is relevant to the study of the economy.
There is more than one approach to understanding how the real interest rate is wrong. New Keynesian analysis dismisses irrational bubbles (its "heart" is rational intertemporal utility maximization).
I think your logic is that Leijonhufvud clearly disagrees with Prescott so he must agree with you. This does not follow.