Grenades don't discriminate
Grenade fragments can travel a long distance and they don't care who they hit. If memory serves, the rated lethal blast radius from a round fired from an M-79 grenade launcher is 5 meters (15 meters for the Vietnam era hand grenade). The fragments travel much further than that.
By "self-inflicted", they mean that he apparently was wounded by his own grenade, not that he shot himself (moron Chris Mathews of "Hardball" to the contrary).