The "bomb" appears to be a Stun Grenade which is less explosive than a standard grenade or fragmentation device.
These devices are designed to be non-lethal when used properly but in this case the grenade detonated inside a confined space, the attackers flak vest, which would have contained the "explosion" and would have caused a considerable amount of damage due to concussive force and heat blast to his body.
Wolff was affected to a much lesser extent due to having two layers of protection between him and the blast.
Stun grenades are constructed with a casing made to remain intact during detonation, containing most of its explosive force and avoiding shrapnel injuries, while having large circular cutouts to allow the light and sound of the explosion through. The filler consists of a pyrotechnic metal-oxidant mix of magnesium or aluminium, and an oxidizer such as ammonium perchlorate or potassium nitrate.
The concussive blast of the detonation can still injure, and the heat created can ignite flammable materials such as fuel. The fires that occurred during the Iranian Embassy siege in London were caused by stun grenades.