I don't think the author's argument is that it won't replace any human labour. Or at least I wouldn't agree with such an argument. But the stronger case is that however much LLMs improve, they won't replace humans in general. In the furtherment of knowledge, because they are fundamentally parroting and synthesizing the already known, vs performing truly novel thought. And in creative fields, because people are fundamentally interested in creations of other people, not of computers.

Neither of these is entirely true in all cases, but they could be expected to remain true in at least some (many) cases, and so the role for humans remains.