I'm not saying Krugman is a charlatan, but answering the question "how can a Nobel prize winner be a charlatan."

It's possible that someone wins a Nobel prize, and then maybe a decade or two later) turns into a charlatan. I think Linus Pauling could be an example: Two Nobel prizes, but later in life he was known for promoting crackpot theories about massive doses of Vitamin C being a miracle cure for cancer. Clinical studies failed to confirm this, and it is regarded as quackery.