The problem is not if the LLM writes secure code. The problem is if you can know and understand that the code is reasonably secure. And that requires pretty deep understanding of the program and that understanding is (for most people) built by developing the program.

I am not sure how it's for others byt for me it's a lot harder to read chunk of code to understand and verify it than to take the problem head on with code and then maybe consult it using LLM.