He predicted that a Keynesian economic policy would fail because it wasn't big enough? So if it fails it reinforces his views, and if it succeeds it reinforces his views. How can anybody be taken in by this?
If the doctor said you need to take 3 pills of antibiotics per day over the next week to clear up an infection and you instead take 3 on the first day, who was wrong if your infection isn't cured? Krugman has shown the work behind his predictions. If you have a more predictive model, the world would love to see it. If you just want to throw up your hands and say all models are BS, that's just denying the predictive utility of models.