This is a good question that we don't have the answer to yet.

Generally, other research has found that the college wage premium grows with age (i.e. a 50 year old with college vs a 50 year old without college degree makes even more in relative term). Thus, if the initial wage premium is driven by the coursework/upskilling due to classes, then we could see the negative impact of less coursework be even stronger in the future.

<- In fact this is what we really want to measure isn't it? Not are you ready for your first job out of college, but did you gain skill that make you a better all around worker.

I think that is what the result may be showing. Even if coursework might not have direct applications, it trains/teaches you in other ways that are applicable to many other scenarios.