You're missing the problem he stated: Code written by humans is usually bad so the model is trained on garbage.
Re:GitHub Copilot
by Jeff_Brown on Wednesday June 30, 2021 @10:56AM
(#27689296)
The proof is in the pudding. I was skeptical too but the testimonials here are impressive.
Based on the FAQ, it looks like some information about how you interact with the suggestions is fed back to the Copilot service (and theoretically OpenAI) to better improve the model.
So, while it may not understand "how to make code better" on its own, it can learn a bit from seeing how actual devs do make code better and theoretically improve from usage.