I didn't notice that it is kind of called out in the post... It's the very last line and left as an exercise to the reader to infer which is almost worse since I would call this a flaw in the implementation of the functions and is being shown off as a sterling example of the capabilities of the language.

The conversion between `[]` and `0` is the flaw that is hard to reason about here and propagates through any code that interacts with it in a way that violates a programmers expectations.