For a better sense of what he means, check out the video go get more detail about some of the complexities.
As a counter-point, a codebase can be well documented and yet be hard to understand if the abstractions are not meaningful to the reader. I can say this without attaching any normative judgment to the quality of the code.
If I recall correctly, Glenn found it difficult to reason about the code because many parts were tangled in complex ways. For example, subsequent processing steps often triggered previous steps to repeat in surprising ways.
To speculate a little bit, these parts of the code may be well-documented down in the weeds, but they could still feel non-intuitive in the broader context if they didn't feel consistent or non-surprising.
As a counter-point, a codebase can be well documented and yet be hard to understand if the abstractions are not meaningful to the reader. I can say this without attaching any normative judgment to the quality of the code.
If I recall correctly, Glenn found it difficult to reason about the code because many parts were tangled in complex ways. For example, subsequent processing steps often triggered previous steps to repeat in surprising ways.
To speculate a little bit, these parts of the code may be well-documented down in the weeds, but they could still feel non-intuitive in the broader context if they didn't feel consistent or non-surprising.