I find it interesting that the book is available about everywhere save for North America according to the author, but the first distances in the text are given in yards, mainly used only in North America. Ok, this is not the exact book text, but still, it looks as a misfit to the audience of the book.
The UK wasn’t on the metric system during the first world war.
Never met a Canadian who used yards, unless they're talking about the grass in front or back of their house.
It comes out mid-August according to https://www.amazon.com/gp/aw/d/0593467426/
In England they still use imperial for some things, like miles, mph and mpg for cars, pints for drinks etc. I wouldn’t be surprised if shooting still used yards as well.
A yard and a meter are basically the same anyways within a reasonable margin of error for lengths less than 5 or 10 meters.
Britain has a strange relationship with metric units, and there are certain things particular imperial units are still used for. Beer and milk is in pints (but nothing else), people are in stones, and how far you can sling something (a golf drive, a rugby ball, or a bullet) would be yards.