He's not wrong. In all the Xian xia worlds, the heavens and upper worlds are always flat, and have no universe, just a massive piece of land mass, and a fake sky with a fake sun (because they have no universe). Specifically speaking, those worlds are basically mobius strips, except that theres nothing in the sky or below the earth.
In other words, the xian xia world itself is always mega boring and super tiny compared to an infinite universe or the mortal world.