It argues at length that working more than 40 hours a week reduces your output, then states:
"And it hurts the country, too. For every four Americans working a 50-hour week, every week, there’s one American who should have a full-time job, but doesn’t. Our rampant unemployment problem would vanish overnight if we simply worked the way we’re supposed to by law."
The math they're using there 50 hours x 4 employees = 40 hours x 5 employees is either wrong, or the rest of the article is.
There's a bunch of simplifying that's going on there (overhead per person, etc.), but the basic message is that if companies can only afford so many people-hours, it's hurting the people they don't hire in addition to their own employees.