The soldiers are roman so it can't be a hapsburg chin joke. The jawbone wielder has a prominent jaw himself, which feels like the set up, but if so I'm missing the punchline.
Getting past the bible part, it's funny because after losing a few hundred guys they're still approaching a couple at a time, and the one guy is being distracting while trying to coach his buddy like, "we can still take him"
I agree, I think that the gist of the joke is in meaningless coaching. Like, yeah, avoiding an enemie's weapon is something one would never guess to do