The simplest explanation is that the exponential model is not a good one (does not correspond to the underlying dynamics), and so the half time value is not an inherent (time-independent) property of the codebase, but depends on its age. It seems to me that in most projects, the code evolves quickly in the beginning and then stabilizes on a slower linear decay. This would explain the observed dependence of the fitted…
He addressed this partly in the article. Even when looking just at the early days of the more mature projects the code churned much less than projects less mature now.