>>> I’m wondering if the impact of a bullet is enough to detonate the explosives?
In order for C4 to explode, there must be a combination of heat and pressure.
You can set a brick of c4 on fire, and it will burn all day on the ground without exploding unless you stomp on it as well.
So yes... I would say this is possible. Bullet contact with metal causing a spark, or the kinetic energy of the bullet entering the jacket combined with the pressure could easily do the trick.
It is less likely that he actually hit the switch with the bullet so that the firing mechanism engaged.
Maybe there was a “dead man” switch on the bomb vest!