Earlier quoted context omitted.
At sufficient levels of intelligence, one can increasingly substitute it for the other things. Intelligence can be the difference between having to build 20 prototypes and building one that works first try, or having to run a series of 50 experiments and nailing it down with 5. The upper limit of human intelligence doesn't go high enough for something like "a man has designed an entire 5th gen fighter jet in his mind…
Exceedingly elaborate, internally-consistent mind constructs, untested against the real world, sounds like a good definition of schizophrenia. May or may not correlate with high intelligence.
They don't have to be. When they aren't, sometimes we call it "mathematics".
You only have to "test against the real world" if you don't already know the outcome in advance. And you often don't. But you could have. You could have, with the right knowledge and methods, tested the entire thing internally and learned the real world outcome in advance, to an acceptable degree of precision.
We have the knowledge to build CFD models already. The same knowledge could be used to construct a CFD model in your own mind. We have a lot of scattered knowledge that could be used to make extremely elaborate and accurate internal world models to develop things in - if only, you know, your mind was capable of supporting such a thing. And it isn't! Skill issue?