Mono. Stereo in audio is based on time (and phase) differences between two separated receivers, such as microphones or ears.
In this case, it would have taken two radio receivers many wavelengths apart to come up with enough signal difference to make any audible difference and be perceived as a stereo difference, after being converted into audio signals. This is a lot like converting into false color when you want to change a received EM signal into one which our senses, at least eyes and ears, can then observe in a familiar format. Radio waves are not a familiar format. Hiss, incidentally, is partly a case of of a lot of different, near-simultaneous, fairly high frequency energy sources scattered along some line of disturbance, somewhat analogous to a line of cars in a long stretch of crowded traffic. Point a L-R stereo microphone pair at a string of traffic and listen to the playback and you can't tell the position of one single car in it unless it is radiating markedly different sound from the nearby vehicles. Record a pin-pong match from the side of the table with the same setup and you can close your eyes and practically "watch" the game in your head. Long transmission line sources are not good candidates for stereo reproduction.
As an exercise, look up "whistlers" in conjunction with, say, "plasma waves" and see what the explanation for those "sounds" is. Lotta research into that stuff.