Flat regarding the universe would normally mean that spacetime is generally flat rather than concave/convex, and it is actually speculated to be so.
But from what he said, it seems he means dimensions (3 of space, one of time). The paired 2's I don't recognize.
Reminds me of a book about a world with a 2-dimensional surface and inhabitants. I think the title was "Flatland."
Regarding the mixing of humans, I am reminded of "Evangelion," an anime and 2 movies where at the end all humans are reduced to a liquid, which retains both their mind and body, and these are all mixed together in the ocean forming one consciousness.