I'd argue the algorithms in the functions you've listed are pretty obvious (they're all wrappers over a single loop), and that knowing how to use these tools together is more important, but I guess that's just subjective

Wrong. By using the libraries of your respective language, you are:

1. Making your intentions explicit and steering away from imperitive code. JS is a high-level language; you don't need to write C-like code in it.
2. Using pre-tested code instead of rewriting it.
3. Reducing code bloat. This solution does NOT need to be 20 lines; you can write it in as little as a single return statement.

This is really bad advice to give to beginners. The only time I can see this advice as being sensible is if none of the tools in the library offer suitable abstractions for the problem and you're then putting a circle in a square hole.

Because it's already there at the end of it.

But it is not correct? The example shows that expected answer is not [3, 2, 1, 7, 9, 8, 6, 4, 5], but something else?

There is this example in description:

Given [[3, 2, 1], [4, 6, 5], [], [9, 7, 8]], your function should return [1, 2, 3, 4, 5, 6, 7, 8, 9].

What does your solution return locally for the above input?

Codewars is the machine

What makes you think that your code works on your machine? It seems to miss an essential piece, it seems it would (almost) never return a valid answer. Does it work for you locally for the arrays from the example? Does it return the same answer as the example shows?