It was a deliberate word choice by the author. "Illegible" conveys that it's difficult to track from the outside. It's a clever analogy: illegible hand writing

It was a deliberate word choice by the author. "Illegible" conveys that it's difficult to track from the outside. It's a clever analogy: illegible hand writing can be read by those who are familiar with the scribe. It's not invisible though: those who work with these kinds of engineers easily see how valuable they are.