If you write a people out of existence as background, rather than as the setup for an exploration of how the world is different without their influence
Never mind "exploration." How about "consideration?"
The problem is not that a carefully researched timeline and 75-page sourcebook are required to discuss the differences. The problem is that wiping out a race of people and then saying "oh, but the world is pretty much the same," is saying those people are irrelevant--they had no influence on history, they made no crucial inventions, their agriculture and towns had no effect on later dwellers in the same areas.
The insult isn't in wiping them out; it's in wiping them out and saying "and that wouldn't be noticed at all by anyone else on the planet."
A book wouldn't have to be *about* that gap in history to be good; it would just have to acknowledge that things would be different. In this case, very different.