Look, one aspect of programming is producing production code that is maintainable, etc.
There are other aspects that are equally as important, and sometimes even more so. Exploring, learning and prototyping are among them, as are "back of the envelope" constructs.
This is a really cool example that shows how far you can go with tiny amounts of code.
Pithyness is actually one way to make code _more_ understandable, given that the reader is familiar with the subject area.
Don't be a snob. The world is a better place because someone wrote this and not worse off.
Since compiler construction is kinda a deep technical field, documenting this in pedagocially sane way would have been a huge task.
I'm happy the author took the effort of writing and publishing this piece, it would be sad if he hadn't just because it's missing an embedded tutorial.
He's not asking you to maintain it. There are references in this thread that explain what is going on...
I write and maintain C++ production code as a day job and some of that stuff is an order of magnitude harder to grok than this (no, it's not documented in any _helpfull_ way either).