> you have to be able to hold very large completely abstract systems in your brain. That's a different skill/aptitude than, for example, holding all the working parts of an internal combustion engine in your mind.

Is it really different though? “Why isn’t this engine working?” [traces through combustion path to find problem] doesn’t require ‘holding all the steps in you mind’ but it does require knowing them and how they interact, whereas “how do I make this engine more efficient?” does require a mental model of the whole engine and how the parts interact?

These seem broadly to me as analogous to respectively maintenance programming and systems development.

Edit: I think the part I don’t understand is why you think the abstract system (the code) is qualitatively different from the concrete system (the engine).