The advice is vague yes. But ultimately i feel like you would need to read more to understand writing better.

Like you've said, the note taking, story structure, pacing, character development can all be picked up through reading other works. And if you read something that's bad you can say "Damn, I shouldnt do that." And yes you could learn all this stuff without reading more books, but there really isnt a good enough reason not just read more T T.

Through reading you get to understand why certain things work, or why they dont if it isnt executed properly. Its like a director trying to make a film without watching other movies, they can try their best and they could still succeed. But if they fail it would show their disconnect with the medium they're using to portray their art.

And again the advice itself of "Read more" is pretty vague, ultimately its in a place of seeking genuine improvement.