4.000.000 J is quite a lot of mechanical energy: it can lift a fire truck 40m high, to give an idea of the damage it can do. An inefficient explosion of a smaller amount of TNT will transfer far less energy to the airplane structure but would be more than destructive enough.
In my opinion the electronics can not have been only pressure based. The altitude where the bomb went off was too close to cruise alt so small measurement errors could have made the bomb fail to explode and if it would explode it would be unclear when.
Taking into account they wanted to film the explosion I think a timer was used. With a couple of sensors (vibration, pressure) it is easy to determine when the plane takes off and wait until the plane is above a predetermined camera position.
Sensor boards that can perform such functions are present in consumer electronics, it is impossible to prevent they get in the wrong hands. To prevent such an event, focus must be on detection of explosives.