To me it seems like a testament to Unix's clean code that McIlroy could pick up existing Unix tools and make a solution so quickly. I think you're actually hitting on many of the highlights of the Unix ecosystem: tons of people read can and improve upon the code to extend its usefulness, or read something written a long time ago (say 40 years ago) and understand assumptions, etc.

This is all theory btw, as I haven't read Knuth or McIlroy's actual code.