"It's like someone from the future..."
Hmmm, it feels like they're over complicating his powers, or the plot. Can't he just be too powerful to control his powers properly or something? We've already got the plot thread about why they're hiding the History of the Hero Reid and His Country, do we really need time shenanigans as well?
That said, I suspect the reasoning for changing their history is simple; They simply won the war and then hid info about the old country, in order to stop people from trying to reform said old country. Reid would be a symbol they could all clamor around, so his history is also hidden. Don't forget this was supposedly a very prolonged war, so they might have just been sick of it, and both sides may have wanted to ensure it doesn't continue.