That does not magically rewrite all previous commit messages, although apparently the improvement is present https://github.com/nim-lang/Nim/commits?author=Araq - most of the total ~25 comments made in the last seven months since December 9, 2021 have more or less legible titles.
That's not what he's saying at all. He's saying that the maintainer now has better commit messages, but there is still a need to go back and produce documentation for undocumented code with inscrutable commit messages. e.g. doing better now does not undo prior damage
On how: The "method" is to just respect the future contributors from the start, instead of waiting for ten years until someone tells you to write better messages.
Why can't it be applied: no idea, writing sensible commit messages seems like a basic necessity if your goal is sustainable project development, but that's just me
Assuming unreasonable, isn't it unreasonable to expect someone to completely break git history for everyone, and or invent time travel?
Seems to be an absurd amount of condemnation over a mistake that did far less than hurting no-one