You don't design things with never ending details though. If your developing a feature there's a limited scope to it. If your designing the direction of a large project you're working with higher level concepts.
The point is that AI is aware of constraints and can manage a round it. You've got to keep in mind the size of a typical software project and plan. Most people aren't writing a kernel. They're writing some backend tool and need a project that fits into a couple of quarters with a handful of people at most involved.