It's not a joke. He didn't even say it was a joke. He said (as quoted on the Wikipedia page for the memo!) that it was “a comment on a research paper that was being prepared by part of my staff at the World Bank” and that it “sought to clarify the strict economic logic by using some rather inflammatory language”.

The closest it gets to being a joke is that it is mockery and derision directed at underlings as a form of feedback on work product.