Before sending the message Sheridan asked Ivannova if she could send the message without the Tragati intercepting it.
My guess would be, it was an optical message rather than a radio message and thus can only be read by being in the path of it (at least in real space).
In Hyperspace the laser (or whatever it was) would most likely then travel in a straight line, which should make it travel along the beacon, which is the logical waiting place for the Minbari cruiser.
Unless Hyperspace somehow refractures light (or whatever the beam was made up from), in which case the message would be shorter range, but could be received in a larger area, probably cone shaped away from the jumppoint.