[jackcosgrove]

From your link, the results section states, "Spence discovered that even if education did not contribute anything to an employee's productivity, it could still have value to both the employer and employee. If the appropriate cost/benefit structure exists (or is created), "good" employees will buy more education in order to signal their higher productivity."

The second sentence flips the causality you describe around. Employees who are already productive buy education to certify their productivity. The education does not necessarily lead to higher productivity. The employer wants productive employees, not certified employees. Certified employees are just a proxy for productive employees. Another proxy, such as aptitude tests or GitHub repositories, could function just as well. These "cost/benefit structures" may very well be more appropriate than formal education because they are less costly in time and money.