That’s a bad analogy because the qualification is being born in a region not acquiring knowledge thru working. an equivalent would be only electrician can write a book on wiring.

Do you mean wire and tool manufacturers? Electrician is mostly a user. Or to avoid flawed analogies - should books on Windows, Office, ... be only limited to a small subset of Microsoft employees? I'd assume the book is for users, what does it matter if the author contributed to the project?

And that's still ignoring that evangelism is also a valuable contribution.

That was exactly why I picked it as an extreme example to show the absurdity of this rule - there aren't any Legionnaires to write the books, so who's allowed to? Assuming we do want the books.