I really did like the author's framing there, but I think there is a different, simpler way to put it:

Why would you think that asking someone else (that is, another human) to write something (and then reviewing it) is the same thing as writing it yourself?

You may trust the other writer's opinions and knowledge, but it will not have the same tone, structure, word choice, understanding, or narrative flow as it would if you were to write it yourself.

And when it's an LLM, you should not trust it's "opinions" and "knowledge", because it does not have either of those things. The appearance of those things is just that, an appearance.