I think this essay is missing a discussion on the theories of what make university graduates better paid. The two main theories are:
1. Human capital. People learn stuff at university which makes them more valuable workers.
2. Credentialism. Universities select (or pass) people who were already going to be valuable.
Now let's look at what we might mean by "valuable":
1. You can fool hiring managers into thinking you're competent - good resume writing skills, good interview skills, an impressive certificate.
2. You can fool your manger's manager into thinking you're competent. All the above, plus you're good at churning out superficially credible explanations and justifications. Some critics of university might argue that they've optimised towards teaching students how to fake competence - 4 years of general education isn't enough to produce a truly competent thinker in a field (not reliably for a large fraction of society) but it is enough to teach them to write a clean-looking essay with proper citations, and to churn out paragraphs with "topic sentence, evidence, analysis, transition" (but not enough time that this will get too stale - you'll still be able to grade the students from A-E).
3. You'll actually be good for your company / society because of your ability to analyze things, domain skills, intelligence, conscientiousness, etc.
We could also talk about general vs domain specific skills, but they're often fairly well correlated. Competence in a domain requires thinking skills, and thinking skills are best built up in some real domain where skills can actually be tested (not just abstract "critical thinking" courses).