The surface area is only imaginary, because there is no thickness to it. Even if it was only half a proton thick, it would still be a third dimension.
By George, I think he's
got it! It is a mathematical representation of three dimensions!
Most drawings are two dimensional representations of three-dimensional reality. Movies, TV, seeing with one eye closed etc. are all ways that you can see only two dimensions, but visualize a third.
The globe of the world is probably the best example of what .13. is trying to tell you. How do you think they make globes? They take a two dimensional piece of paper (assuming the thickness is negligable), put a map on it, and paste it to a globe. The map is still only two dimensional, but it is wrapped around a three dimensional object. If you peeled the paper off and flattened it, it would be essentially a two-dimensional map again.
Now, visualize the same thing but instead of a sheet of paper around a globe, imagine the skin of a balloon. The surface of the balloon approximates two-dimensionaliity, since the thickness is negligible.
The process is probably similar with a baffoon.
Quick, while you've got that model in your head, imagine the expanding universe. You are not considering the air inside the balloon or the air outside the baloon, but only the two-dimensional representation of the skin of the balloon itself.
Whew!
Edited to add another example.
Think of how positions are located on a map of the earth. They use latitude and longitude. That is what your GPS shows you. Sure, there is elevation, but for the purpose of location, lat and long are enough. It is a two-dimensional co-ordinate system for the surface of a sphere.