I actually agree that government spending can boost GDP. Where Krugman doesn't get it is that we've already distorted markets so much since 2008-2009 that a burst of government generated demand will not move the needle in a sustainable fashion. A $1 trillion government program will just get drained into rising asset prices - leaving incomes and the majority of individuals in largely the same spot as they started financially.
Krugman is fighting an old war.
Our current economy is not encumbered by too little demand - but by too much supply from past capital misallocation.
Our addiction to QE has made problem far worse. Asset markets are exploding in price higher due to central bank intervention while incomes are not.