I disagree with the author. Text explaining a "why" are often contemporaneous. To put it all in the source text would be too bloated and too prone to bitrotting

I disagree with the author. Text explaining a "why" are often contemporaneous. To put it all in the source text would be too bloated and too prone to bitrotting into misinformation to be useful. To increase concision to the point where bitrot is unlikely loses useful contemporaneous detail. So, I prefer concise comments that are highly salient and unlikely to turn into misinformation, usually about key details on a mechanism where subtle mistake is more likely than the norm, and detailed commit messages on contemporary justification. Also, the author mentions the commit messages being "probably lost." I'm not sure what the author is doing to get themselves into that situation. I work on code bases with good commit messages. I use emacs's vc-annotate with regularity.