An LLM most definitely cannot spit out robust tests or thorough documentation. It can spit out some tests or some documentation, but they will not cover the user perspective or edge cases unless those are already documented somewhere. That's verified by both experience and just thinking about it for two seconds.
The sanding down you refer to is what generates those tests and documentation.
> but they will not cover the user perspective or edge cases unless those are already documented somewhere
Are you suggesting that LLM's can't test for people who use screen readers? Keyboard only users? Slow network requests?
You're acting like the issues an app faces are so bespoke to the actual app itself (and have absolutely no relation to existing problems in this space) that an LLM couldn't possibly cover it. And it's just patently wrong.