I think it's like saying if you learn how to maintain a submarine, have you become a good swimmer? Or, if you know how to maintain a submarine do you understand fluid mechanics?
Ok let's take one step back and look at the parent:
> I don’t see how this can be true under any definition of computer science.
Let me arbitrarily copy one definition of computer science, from Wikipedia:
> Computer science is the study of computation, automation, and information. Computer science spans theoretical disciplines (such as algorithms, theory of computation, information theory, and automation) to practical disciplines (including the design and implementation of hardware and software).
I think that designing an OS counts as computer science, at least under "some definition of computer science". And learning how to maintain an OS is a step towards understanding how it is designed.
Of course, maintaining your OS does not teach you Javascript. But Computer Science is not limited to Javascript. And I have seen many developers distribute libraries without having a clue about how package management works, which results in a big mess. And then they complain about the tools ("CMake sucks, it's not my fault"), where actually they just don't have a clue how it works below.
I see computer science as the discipline that makes the whole computer work. Because one can't be bothered to understand anything below their favourite framework doesn't mean it doesn't count as "computer science".
