Good code consists of easy to use abstractions. In general Chris's blog is dedicated to poking bad abstractions and giving good examples. `substring` is objectively bad.
You (and other commentators) literally arguing that keeping staff in the head or making some documentation or notes or forcing yourself or others check low level implementation details are better than making one trivial fix and forget about it. I find it really amusing.
Good code consists of solving a problem simply and efficiently. You can have a great program which is concrete. Abstraction is a tool for managing complexity, not a goal.
I would really encourage writing a large program C or assembly, as a practice of how to deal with programming problems without immediately wrapping things in an interface. A lot of things that feel like producitivity end up being wastes of time.
While it is "obvious" that it is correct, it is, in fact, neither obvious nor correct if A. you ever subtract from then len field, and B. you use "len != 0" as a check for "is string non-empty". This program does both things quite a lot and it requires a conscious check to re-assure yourself that no, in this particular program, the patterns of acitons A (subtractions from the len field) never break the property B (non-zero len means that the string is not empty). But any minor modification may accidentally break this harmony if you are not aware that it is required to hold.