This is one of those very unfortunate situations where good people make bad decisions, probably in haste. It's an occupational hazard in blogging, often to be overlooked, but one should admit it when one has jumped the shark, as Krugman clearly did. He obviously trusted ordinarily competent Nick Rowe to have it right. But there is always a risk in not consulting the source, and Professor Krugman fell victim to this freshman error.
Read it at
Relentlessly Progressive Political Economy
Hubris leads to incompetence: the Rowe & Krugman edition
by Travis Fast
(h/t Travis for providing the link in the comments)