Well, I guess the example is more confusing than clarifying. I used that for a case of pushing the envelope. When Facebook needed a better solution for their feeds, they invented Reactive, and I was saying that one would need to know compiler to build JSX and need to graph to build the optimized virtual DOM. Yes, nowadays we just hire Reactive users, but my point is that in the future we may have another moment that we need to invent something, and as a company founder I sure would like to hire the author of Reactive or the like.

To use an existing one.

And tools are best used when they are understood.

Hiring, like many things - including engineering - is about tradeoffs.

Is someone more knowledgeable "better"? Sure. But sometimes it's about getting a specific thing done, and if you're hiring someone to improve a web thing that uses React, maybe you don't need someone who understands compilers and the kernel and how the underlying hardware works and all that. Maybe you can spend a bit less and get someone who will do a perfectly adequate job.

I don't know anything about quantum mechanics. Does that mean I don't understand how computers work?

The common advice is to understand one level above and one level below where you operate.

For example:

If you build computers, you should understand how the individual components are implemented.

If you build computer components, you should understand materials science and electromagnetism.

If you study electromagnetism, you should understand quantum mechanics and relativity.

How many react developers understands compilers & graphs? I'd wager that it's way below 10%.

Also why do you need knowledge on compilers and graphs to create React?

Firstly, React is not compiled. Secondly the graph or tree or whichever aspect can come naturally when you come up with the idea of how it would be best to maintain JS apps.

In fact most important experience should have been grinding put tons of unmaintainable JS apps to come up with something like that.

I wouldn't argue with that.