@LilyWhite
Well, it's not impossible in this world. 😋
I suppose it comes down to suspension of disbelief. You can find it ridiculous while I don't even bat an eye at it. The author has built a world in which the story takes place and not everything will be liked/understood/accepted by everyone.
But, like you said, the story itself is good enough where some of these extra details can be mostly disregarded. The fact that the empire was founded 7000 years ago doesn't seem to be a major plot point, but a mere device to establish that this Etai group has been around for a long time and has deep rooted disdain for the empire, which will likely lead to something more plot relevant later on.