Five stars for great, in depth information on topics like how Amazon bestseller rankings work (and how you can give your own book a shot). Four stars because the info is so out of date (2013). And two stars because the info may no longer be that important except to a very few, so three stars overall.
Here's the issue. Gaughran explains that the big bucket (Literature & Fiction) has 620k titles. But if you drill down: Lit & Fic > Genre Fic > War you are in the pool with only 5k titles. Far more likely to hit Amazon's recommended titles, bestseller lists, popularity lists etc. (and all these will boost your sales) if you pick the War genre (assuming war is your topic).
Here's the other issue. That was 2013. Today Lit & Fic sports 3.5M (yeah, million) titles. Little War has 34k titles. Your very good war drama (it's very good, right?) will hit top-ten in sales by beating out 33,990 other war dramas (some of them may be pretty good, too).
Me? I've got a romance. Best one written since Jane Austen dropped her pen, of course, but still... (Only 1.2M other romances to compete with. Yeah, million.)
P.S. That's about six-fold growth in both Lit&Fic and War. Six-fold growth is 80%/year. Mid '17 will have, at that rate, 6M titles in Lit&Fic, War will be over 60K. Someone quipped, 'Writing is like vagrancy, but it pays less.'