(Attempting to) writ(e)ing a language is indeed very instructive. No need for it to be useful, the attempt alone serves its own purpose.
I never got round at attempting a compiled or scripted language so a thumbs up at this guy. I just did a very basic assembler + emulator but even that was very insightfull.
From that point of view, the guy may have learned a lot (one hopes).
The thing that set me off dubbing was the fact that, apparently, after a few years of coding, he (still?) seemed to be amazed at 'the amount of combinations one can make using mathematical operators'.
I would have assumed that after, say, a few weeks of programming, you don't even *think* about those kinds of things anymore, right?
As you learn your 'amazing discovery threshold' rises - it takes more for you to think "wow, I didn't know you could do things like *that*!?".