It's more accurate to say that Krugman and many other contemporaries think inflation is rising prices and can have causes other than monetary expansion.

The original meaning of "inflation" referred to an expanding money supply, which causes price increases.

The answer to your question will depend on whether or not you think they're genuine in their stated beliefs. I believe Paul Krugman is just a highly talented partisan activist and he doesn't actually believe what he's saying. His goal is to justify whatever his side is trying to do, which just so happens to involve printing tons of money. When Republicans are in power he makes completely different arguments. As such, he's just using sophisticated sounding economics jargon that doesn't actually make sense, in order to persuade people that his side's policy makes sense.