Good read. I would especially want to know more about the unpredictability of the red code/unhealthy code. The difference between healthy code and red code regarding implementation time is astonishing.
From the paper: "Could it be that changes done by the main author are faster and more predictable than corresponding changes made by a minor contributor? We suspect that author experience could be a factor that impacts the predictability of changes to low quality code."
If that turns out to be the case, then it would highlight organizational dimensions of low quality code: key personnel dependencies and on-boarding challenges.