They eventually agreed with his plan and decided to gave him what he could use to achieve it, but it wasn't a thermal bomb. It was an electroshock shock. It would fry the circuits of anything using electricity, including the nervous system of humans.
Any human but by it would be paralyzed for a short while. That's for the truly strong individuals. As for those who are weak, it will fry their brain and kill them.
They gave him this alternative because they don't want his bomb to bring down the building. They don't want to pay for the whole building, and they also don't want to kill the human plants.
They intend to rescue the human plants. According to their investigations, the electroshock bomb won't kill them because of their regenerative ability. They should only be paralyzed while the guards are rendered catatonic and the turrets are fried.
