Every component of a deep neural network is understood by many people, it's the interaction between the numbers trained that we don't always understand. Likewise, I would say that we understand the components on a CPU, and the instructions it supports. And we understand how sets of instructions are scheduled across cores, with hyperthreading and the operating system making a lot of these decisions. All the while the GPU and motherboard are also full of logical circuits, understood by other people probably. And some (again, often different) people understand the firmware and dynamically linked libraries that the users' software interfaces with. But ultimately a modern computer running an application is not through and through understood by a single human, even if the individual components could be.

Anyway, I just think it's fun to make the thought experiment that if we were here 40 years ago, discussing today's advanced hardware and software architecture and how it interacts, very similar arguments could be used to say we should stick to single instructions on a CPU because you can actually step through them in a human understandable way.