technically speaking a mono sound is a file containing 1 channel, whereas a stereo file contains 2 channels. if a sound contains the same information in both channels it will sound as if it is a mono file but it will still be a stereo file...
It's a mono sound panned (spread) in a stereo mix. But since L and R share the exact same information (center), there's no stereo information which is the difference between the L and R channels.