That's simply false. Even if LLMs don't produce correct and valid code on first shot 100% times of the cases, if you use an agent, it's simply a matter of iterations. I have claude code connected to Playwright, context7 for docs and to Playwright, so it can iterate by itself if there are syntax errors, runtime errors or problems with the data on the backend side. Currently I have near zero cases when it does not produce valid working code. If it is incorrect in some aspect, it is then not that hard to steer it to better solution or to fix yourself.
And even if it failed in implementing most of these stages of the plan, it's not all wasted time. I brainstormed ideas, formed the requirements, specifications to features and have clear documentation and plan of the implementation, unit tests, etc. and I can use it to code it myself. So even in the worst case scenario my development workflow is improved.