Well Shiro's world doesn't exist like how we think of our universe, the world which the characters inhabit isn't a planet, it's not even spherical, and then you have the realms, which are basically 3 layers, which are separate from each other spatial and temporally, but still have the same time.
Celestial objects are more so systems, which is why the sun rises and falls at the same time in all realms.
But yea the outer space not existing is an interesting statement, don't think we can take it too literally because we know that if you can bypass the restrictions,
you could physically get to the Sun, though you could maybe make some weird argument that they don't exist physically at all, but only as systems/laws.