Imagine having the entire western world backing you with billions of dollars and the most advanced murder machines in human history and you get owned by a fuckin rock.
I don't think you need to pierce anything to fuck someone up with a rock. People get concussions while wearing helmets, this is just more extreme.
Medieval fighters used maces against armour. I'm not sure how well a combat helmet works as a hard hat against thrown debris (since they're designed for bullets/shrapnel rather than blunt force trauma), but I think getting concussed enough to die is possible. Or maybe his neck got broken, idk.
yeah i don't care what helmet i'm wearing, i'd still rather not get hit in the head with a sling stone or a big rock dropped from above or something, it's still going to be a bad day at best