[wglb]

Those are shortwave, that has vastly wider propagation than the 833 khz that the Bridge radio company was using.

For the broadcast station, one key to very long distance is the 250 foot towers.

[Johnythree]

The key would be the lack of interference, both from electrical noise and from other stations.

Radio Amateurs are even today communicating world wide on the 630m MF band using much lower power and very modest antennas.

[wglb]

Yes, however, they are likely using new techniques such as WSJT and related modes, that have the capability to receive many tens of DBs lower signals.