"Legibility" is a distinct reference to Tom C. Scott's Seeing Like a State, and the tendency of states (or other large institutions) to attempt to impose legibility on complex problems, domains, or systems, usually in an attempt to manage or govern them, but often destroying that which makes them valuable in the first place:

<https://en.wikipedia.org/wiki/Seeing_Like_a_State>.

It's not invisible, it's incomprehensible to most would-be employers because the information is not in a form that they can understand. That's not the same as invisible.

Passing some leetcode challenges is legible to employers, it puts applicants in the same buckets and measures them in (roughly) the same way. But it's not a good measure of actual talent. The real measures, as discussed in the article are out there but illegible to those employers.

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.

It's not invisible, because some people can see it. It is illegible because the leadership of a large company won't know how to interpret it. This particular usage of "illegible" has been around for awhile, but is probably best known from the book, "Seeing Like A State":

https://www.amazon.com/dp/B07D2HZXB4/