In the context of what rioters were confirmed to have been doing out there, nobody should be expected to wait until something hits them, to find out how dangerous it is. Throwing anything at anybody is assault if it touches them at all or forces them to move quickly out of the way to avoid being hit. And when crowds of people are throwing molotov cocktails and other items capable of causing death or grievous bodily harm, it's not unreasonable to defend oneself from the throwers. He was also being chased by rioters, and very reasonably believed that they intended to cause him grievous bodily harm.