Identifying Flaw in Argument on Four-Day Work Week

In a recent lecture, a professor argued that implementing a four-day work week would significantly boost employee productivity. He referred to a pilot program conducted at several tech companies, where employees reported higher job satisfaction and completed tasks more efficiently. Based on this evidence, the professor concluded that adopting a four-day work week could enhance overall workplace efficiency. However, some students questioned this conclusion.

What flaw in the professor's argument is most apparent?