To add to what I said in another post - I have found this very difficult to apply to code. Military skills, while underestimated, virtually always have very, very clear steps. Perhaps this is also true of specialized medicine, though probably not outside of quickly troubleshooting certain conditions (stroke) or certain common procedures (clear airway). Basically, combat medicine.

In programming - good luck, literally everything you work with is new, all the time. I guess that's why FAANG tests on useless algos - it's at least something you can memorize and pretend it applies to everything.

Maybe I am just using excuses, but I have never used things I wrote articles on: RxJS, Angular Route Resolvers, Modern CI/CD, Web Components - all of these things either died or I used them once in my career and even the articles themselves are useless.

Not necessarily. To teach somthing you need to understand it very well. The core of it. The essence of it. When I studied I was helping others with the material. I felt how much that improved my understanding of it.

So they might have meant that you don't just need to be able to teach it. You'll understand it only after you've already taught it to someone.

I think it's also important to understand where the practical understanding cut off is. When you tell someone to npm i some_package, you need to pretty much tell them not to worry about how that works under the hood. When you are teaching someone to load a rifle (to follow the initial example used above), you need to pretty much cut them off when they start asking about the composition of modern smokeless powder vs black powder or something. My point is that knowing how deep NOT to go is key to practical knowledge or teaching it.