I agree that for many applications the code written by an LLM can be good enough, as proven by the many commercial applications that contain even worse code.

However, anyone who uses an LLM must remain aware of the limitations of this method.

There are many features of a program that cannot be tested exhaustively and which must be guaranteed by its design. When you do not understand very well the structure of a program it may be difficult to decide what must be tested.

With performance, the confidence in what an LLM produces is even lower, because it is unlikely to know if you have really reached a performance limited by hardware. Obtaining a performance better than a previously existing program does not prove anything, because most existing programs are likely to have a performance much lower than possible.

In many cases you just want a performance good enough, not the best attainable, so you can be content with your LLM-generated program. But you must not fool yourself by believing that this is really the best that can be done.