A president's most valuable asset—with voters, Congress, allies and enemies—is credibility. So it is unfortunate when extreme exaggeration emanates from the White House.

All presidents wind up saying some things that make even their own economists cringe (often the brainchild of political advisers unconstrained by economic principles, facts or arithmetic). Usually, economic advisers manage to correct these problematic statements before delivery. Sometimes they get channeled into relatively harmless nonsense, such as President...