And how do you actually know that those 20k line of codes have no glaring bugs, or bugs that you can find yourself, or be able to understand it completely at some point?
As for coworkers, I would really try to get them to work in chunks smaller than 20k loc. But at some point you have an expectation that coworkers will be accountable for their area of responsibility. If there's a bug in their code, they're expected to fix it. If there's a bug in the AIs code, I'm expected to fix it....
The spec and the test are your human contribution.
To have useful tests, you must write the APIs for the functions, and give examples of how to wire up the various constructs, and correct input/output pairs.
Implementations of those functions that pass the test now have significant constraints that mean you understand a lot about it.
First you write the tests, then you write code until tests pass.