Even then, isekai just means "another world" and has nothing to do with reincarnation inherently (someone can die and be reincarnated into another world, or someone can simply be transported to another world, and both are isekai scenarios, but being reincarnated into the same world isn't an isekai just like being magically transported from Tokyo to Los Angeles isn't an isekai), so "native isekai" is still a dumb term in any scenario.
I understand why people get isekai vibes; this is about as close as you can get, where the world has changed so much in a couple millennia that it's practically a different place and it thus it follows a lot of isekai tropes. Regardless, the world being the same one is still very integral to the plot. If this is an isekai, then Lord of the Rings is an isekai because two thousand years pass between the Ring being dropped in the Anduin and Déagol fishing it out.
And before someone misunderstands me and makes a dumb reply, I'm not saying this is bad or that it doesn't feel like an isekai, just that strictly speaking it isn't one and that "native isekai" is a weird oxymoron. It's obviously checking a lot of boxes, and I think it's pretty fuckin' hilarious.