So, they go to all this trouble ... why didn't they just set up a bi-directional laser, in other words, laser on each end, camera on each end, relatively close to one another, at the same height above the ice. That would definitely rule out refraction, since you can't refract down in opposite directions. Refraction requires an increasing density gradient, and so if it's increasing in one direction, it's decreasing in the other.
But, wait ... Dr. John D did perform such an experiment, and definitely proved the flatness of the earth, ruling out refraction.