What I don't get is the risk of an accidental signal triggering it. The system as described sounds similar to some existing tech used to communicate with submarines when they are under deep cover... presumably those comms come with both strong encryption and strong authentication off the rack. seems unlikely that random noise would be a concern for a trigger...
A more likely rationale for a fixed time line would simply be a concern on the life of the battery in the receiver, and the diplomatic fallout that would happen if the battery died, and an intact detonator was recovered on a later routine inspection that could be traced back to the USA.