What is educational productivity?
Inputs in time/money relative to outputs.
In this case, comparing 12/13 years of schooling (high school without/with KGN) versus 16-18 years that would include a 4 year degree.
The definition of a low-skilled job is not tangible. For example, bank clerks can be people without an education, but college education helps.
Please, enough with the hair-splitting. Yes, there are many jobs that are low-skilled. I have worked some of them: landscaping, residential irrigation, warehouse work, truck driving. The article in the OP uses two examples: wait staff and cashiers.
More generally, if your average person can learn the job with short or moderate OJT, (to use the BLS's verbiage), you're talking low skilled.
Meh, "bank clerk" is not on the list that I could find, but plenty of related jobs. In the list's terminology, it looks like Short or Moderate-term on-the-job training.
Some sort of college education
might help a bank clerk. If one avoided any quantitative courses, I doubt college would help at all.