You don't really know if the system design you've architected in your mind is any good though, do you, until you've actually tried coding it. Discovering all the little edge cases at that point is hard work ("a total slog") because it's where you find out where the flaws in your thinking were, and how your beautifully imagined abstractions fall down.

Then after going back and forth between thinking about it and trying to build it a few times, after a while you discover the real solution.

Or at least that's how it's worked for me for a few decades, everyone might be different.

He did not say he does not iterate! And it is much easier and faster to do when an LLM is involved.