As a bonus, porting Doom to it should be "trivial".
A specific type or area of developers, I'd say. There are many types and not all of them require understanding sizeable code bases to do their work well.
Done right, you may not know specific lines or chunks of code by heart, but much like a tuned-in company CEO, you have eyes and ears on the ground and retain global oversight and insight of the project itself. For specifics, you can learn what you need as you need it. If that means knowing how every single module works, that's just a conversation with your agent.