True, forces on the lid start to compare to the same force behind the bullet. The lid isn't the only thing in consideration. The BBs, the nails, the metal shards, the fine components of the bomb are much smaller area than the lid of the pressure cooker. 100 psig behind the BB, compared to 20,000psig behind the BB? The force behind the bomb components are much smaller compared to using a better delivery vehicle capable of higher pressures. This points to poor technical sophistication, and an amateur understanding.

The maximum velocity of components from a bomb is dependent on the pressures generated during bomb's explosion/combustion. If it's topping out at 100psig, that's a mere fraction of what it's capable of.