I don't think that zombies are smart enough to throw rocks.
But who cares? There's a thousand of 'em. And attacking with weapon 1 rocks, you need to hit with 6 extra shifts for a take-out. Against Great defence.
You just can't kill that many with dropped rocks fast enough for it to matter.
As for the strong dude, he has a similar problem. Plus, I doubt he'd survive being mobbed.