LLMs definitely write worse code for LLM consumption than humans can. In my experience your claim can't be further from the truth. I can get much further with an LLM starting from a great codebase than I can starting from a vibed codebase.

And I can do even better than that if I design the codebase specifically with LLM coding in mind, making choices that make it hard or impossible for the LLM to make certain categories of error it tends to make, and make it easier for the LLM to observe the results.