Economics is a very broad field. There are many parts to it. Krugman actually made a very important breakthroughs in a very narrow area. Unfortunately he feels it necessary to talk about all of the others... like an astrophysicist who made a breakthrough in understanding the pattern to solar magnetic fields commenting authoritatively on Ocean currents and assuming that they are dominated by local magnetic fields.
You are exaclty correct...once someone is credentialed they believe they are experts on everything.