Looking at own git logs (for code, not prose), I notice git is very happy to add and delete lines even if just some characters on those lines have changed. Is it not the case that the author of the article has simply edited 90% of the lines in his book, rather than rewritten them completely? That seems much more likely to me.
Git doesn't really have good statistics (I think) for e.g. token changes (it would need a tokenizer for that). There is however --word-diff for showing a diff in words instead of whole lines; it's probably possible to count those as well, the output syntax looks parseable enough.
On the other hand, editing takes a lot of time per word changed, because you have to consider the whole sentence or even paragraph to make sure the grammar is still right, you're not repeating things, and you haven't removed any necessary context. So maybe a bit of overestimation is good.