Ah I think I see it now. Topology is a bit complicated approach. If you have a sphere with a pole defined to be infinity(because a sphere has no beginning or end?) you can transform it into a flat unbounded plane. This because the flat unbounded plane can be wrapped around into itself at infinity to form a sphere.
e.g.
https://en.wikipedia.org/wiki/Riemann_sphere
And the plane of Nirn might extend to infinity because there is nothing in the map of Tamriel that says otherwise and thus it could be topologically the same as a sphere?