[Confusion]

  I think the fact that college freshman are capable of
  grasping it in a semester provides some evidence against
  your claim that you need "much, much more knowledge of all
  kinds of things".

I would say they probably grasp it as much as I grasped quantum mechanics after the first course: I was able to go through the motions and explain the concepts involved, but that doesn't mean I actually understood it. I would claim that at some later point, after more courses, labwork and discussions on the subject, I did understand certain parts of it. Much of the additional knowledge and principles I learned are already implied in decent basic texts on the subject, but I simply wasn't in any position to 'expand' those implications.

  There's tons of stuff you can do with this foundation,
  including writing DSLs.

However, writing your first DSL on that foundation requires you to reinvent many wheels, some of which you will reinvent in a square way that first time. Knowing how to write a programming language in Lisp doesn't tell you how to write DSL's, like having a Ph.D. in English means you know how to write a novel, without you actually being capable of writing an good novel. They would still benefit from reading books on 'writing novels' by people that have practical knowledge in writing actually good novels, even if those books were written by authors whose genre they dislike and the style of whom's examples they dislike.

  the distinction between internal and external DSLs?
  Completely unnecessary.

If everyone would program in Lisp: yes. However, people write DSL's in different languages, for all kinds of reasons you may or may not agree with. That means books on writing a DSL in other languages than Lisp simply have their place. And may also be valuable for those writing DSL's in Lisp, for the reasons mentioned above.