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Does "git grep" use a system grep or does it implement grep on its own?

That seems to be the "command infrastructure" for the grep builtin. The actual grep engine is in https://github.com/git/git/blob/master/grep.c.

The article has an answer to that. I'm not sure if you're challenging the article's answer or if you just missed it...

Oops, missed that part of the article, thanks!