The funny thing is, I actually can mine ore, smelt it into iron and forge it. That doesn't mean I can explain how that works (monoxides in a fuel binding to oxides in iron ore to separate iron from slag, aligning grain and changing durability by heat treating, etc). Maybe that doesn't matter if all you want to do is produce pig iron, but if your specific engine design requires a specific grade of alloy, now you need to fully understand metallurgy.

Take it a step further and the engine is built, but you need to make a modification (forced induction, race fuel, etc). If you don't want your engine to blow up, now you need to understand air/fuel mixture. Sure, I can do something with a simplified understanding. But the lack of a complete understanding will leave me with a blown-up engine.

Does it follow that if I understand metallurgy and internal combustion engines, but can't simplify it, that I don't understand it? Of course not; the entire premise was that I already understood it. Simplification does not show you understand something. It can only provide a shallow, incomplete understanding.

So you don't need to be able to simplify to understand. You need to understand to understand. You need to simplify to provide a simplistic, shallow view.