I hear this a lot as a reason to accept that only the robots have read the code: "but it's too much code for one person to understand."

Yes, but it's not ever too much code for a team to understand, or a community. The question is whether you can build a team or community with enough trust and integrity that two things are always true:

1. Alice understands the fluxtron renoberator module she wrote.

2. Bob can also shift his attention there and understand it if necessary, because Alice writes clean code that was reviewed by John and Sally.

You can have Alice, Bob, John and Sally all be robots, and you can decide to trust the robots, but (so far anyway) it's a choice, not an inevitability.