Now I found "the rest of the story" from the person who faked it.
Interesting and disturbing analysis!
Elsewhere, someone wrote,
I don't know whether the imposter intended this, but his joke has a stroke of brilliance.
By making the outrageous comment, he indirectly compels Krugman to respond to it, and say for the record whether or not he accepts it. The beauty is that no matter what Krugman says, it will make him look like an idiot.
If he says he does not accept it, then he contradicts his own worldview, because the comment is exactly in line with Krugman's orthodoxy. It wasn't an advocacy, it was simply an argument of economics.
If he says he does accept it, then he will just reconfirm what everyone already knows. That he's crazy.
If he fails to say he rejects or accepts it, then that will insinuate that he has something to hide, which conveys the impression that he secretly accepts it and is too embarrassed to explicitly admit it.
Since Krugman seems to imply in the NYT blog that the fake comment is "really stupid [and] outrageous," (although we can't be sure, and I bet Krugman carefully worded his column so that he doesn't explicitly say the comment is really stupid and outrageous, but at the same time denying he said it), then the imposter has succeeded in getting Krugman to contradict himself.