From the perspective of someone who has written a fair amount of Hetalia, I think the aspect I would want to distinguish here is the idea of human characters as avatars. The characters in Hetalia are not human, for all they appear to be so. They exist on an entirely different timespan from humans, a fact that the Hetalia canon does explore on occasion. It's also suggested that the Hetalia characters don't really have free will of their own, at least when it comes to decisions made in their name or things that happen to them (going to war, getting "married" to another nation, etc.). This book sounds like it takes pre-existing humans and makes them avatars/personifications even though they're still humans, with human lifespans and decision-making processes -- which presents an entirely different set of complications.

To some extent, I think I prefer the concept of "this avatar is a human-looking personification akin to a Greek god, however much of a hand-wavey walking stereotype" to "this actual human character gets to be the avatar because they are the Single Most Representative Human of their locality". Admittedly, this is probably a gross oversimplification of the book's idea, so apologies if I've missed the mark.