You're right, it does address it, but mostly by a bunch of handwaving. It does not address my central point of why employers aren't luring away these highly producing yet underpaid workers, or why more businesses aren't being started to employ them if there's a surplus of them.

Furthermore, their argument continues to assume that the increasing productivity of an entire business can be assigned more or less equally across all of its employees. However, this is not true by inspection - how did the janitorial staff's productivity improve? The productivity increase of the entire business may be the result of the efforts of a very small number of the employees, such as one who improved the manufacturing process.

And lastly, it assumes that being better educated means more productive. This is not necessarily true if one is in a job that does not make use of that education. For example, it's hard to see how having a degree makes one a more productive cashier.