No. It means the surface distance using the Haversine formula.
I'm not sure what you are trying to ask in regards to "how does it express distance in radians" but if you mean how are lat/lon angle values and deltas converted into radians it is: PI * angle / 180.0. For the full Haversine formula, check out this link for implementations in 84 languages: http://rosettacode.org/wiki/Haversine_formula
Without looking at their code it is impossible to say. But given that they use WGS84 and talk about an "earth-like sphere" it is highly doubtful. I'm guessing they use Haversine (so distance calculations are only an approximation).