I've read Frank's previous book, "Light of the Stars: Alien Worlds and the Fate of the Earth", but not the one that came out 2½ months ago (which was not mentioned in the story). I'm not particularly a fan, as he tends to draw sweeping conclusions from just a few uncertain pegs of knowledge. I don't think we know exactly what's possible. Can we even measure the O2 abundance in exoplanet atmospheres that accurately, to exclude those that don't make his cut?
On edit: in thinking about this I remembered the oxygenated fuel nitromethane, used in super-charged "fuel" drag racing and the little model airplane engines that were around when I was a kid (maybe still are). The Top Fuel and Funny Car racing these days utilizes fuel pumps that put more than a gallon per *second* into the engines, to produce something north of 8,000 HP (though I'm not sure how they measure values so high), the point being that lack of there not being more oxygen in the atmosphere doesn't seem to be the limiting factor.
So someone/thing might be able to make a go of it with less O2 than us (or <18%) if they used something that contibutes some of the oxygen to the combustion process. There might still be a gap in getting to where such things were discovered or able to be made in the first place. But maybe if your planet only had, say 15% O2, it might not be an insurmountable obstacle.
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