When LLMs generate an appropriate program from ambiguous requirements, they do this because the requirements happen to match something similar that has been done previously elsewhere.

There is a huge amount of programming work that consists in reinventing the wheel, i.e. in redoing something very similar to programs that have been written thousands of times before.

For this kind of work LLMs can greatly improve productivity, even if they are not much better than if you would be allowed to search, copy and paste from the programs on which the LLM has been trained. The advantage of an LLM is the automation of the search/copy/paste actions, and even more than this, the removal of the copyrights from the original programs. The copyright laws are what has resulted in huge amounts of superfluous programming work, which is necessary even when there are open-source solutions, but the employer of the programmer wants to "own the IP".

On the other hand, for really novel applications, or for old applications where you want to obtain better performance than anyone has gotten before, providing an ambiguous prompt to an LLM will get you nowhere.