If they go far enough with it they will be forced to understand it deeply. The LLM provides more leverage at the beginning because this project is a final exam for a first semester undergrad PL course, therefore there are a billion examples of “vaguely Java/Python/C imperative language with objects and functions” to train the LLM on.
Ultimately though, the LLM is going to become less useful as the language grows past its capabilities. If the language author doesn’t have a sufficient map of the language and a solid plan at that point, it will be the blind leading the blind. Which is how most lang dev goes so it should all work out.
Proper code review takes as long as writing the damn thing in the first place and is infinitely more boring. And you still miss things that would have been obvious while writing.
In this special case, you'd have to reverse engineer the grammar from the parser, calculate first/follow sets and then see if the grammar even is what you intended it to be.
Author did review the (also generated) tests, which as long as they're comprehensive enough for his purposes, all pass and coverage is very high, means things work well enough. Attempting to manually edit that code is a whole other thing though.
At least for me that fits. I have quite enough graduate-level knowledge of physics, math, and computer science to rarely be stumped by a research paper or anything an LLM spits out. That may get me scorn from those tested on those subjects. Yet, I'm still an effective ignoramus.