@Phan_Minh_Khoi Well to be fair there's still a bunch of rulers of the past that we don't even know about and we need all of today's technology to help us to know what we know today (and we even with that we can still get things wrong), and also about the political change in 1000 years, just look at Medieval Europe, that place could had different regional powers throughout an entire millennium, so it's not that farfetched for having radical changes in less than a 1000 years, just look at some map video of the history of Europe and see if the political landscape didn't change in less than a millennium
And about forgetting something historical just search about the Burning of the Black Wall Street - Tulsa, OK. That was something that happened in 1921 that was erased and forgotten from American history, that only a few knew that it had actually happened
Now for everyone else. Is this an Isekai? Yes and no, Yes, because the Voids are from another world coming into that world so from them it's an isekai. And no, because from the protagonist's view it's his world but in the future