Kevin Hassett, chairman of President Trump’s Council of Economic Advisers, caused a brouhaha by claiming last week that the corporate tax cut enacted last year has “about paid for itself.” I told Bloomberg it is a little premature to say that, and critics have asserted that even a Trump economic adviser disagrees with Mr. Hassett. But I’ve looked more closely at the numbers, and it turns out he is almost entirely right.

Compare the August 2018 economic forecast from the Congressional Budget Office with the one from June 2017,...