The thing was, the head of the spear was wider than the link of the chain, hence why it got stuck in the first place. The edge of the spear’s point was sticking out past the chain’s width and could still cut the man, even as Brennus drew back and dragged it along his groin, stepping backwards to hasten its retraction. So, while the chain did indeed reduce the damage he could have taken, it would not negate it entirely. 

Not that it mattered, doing damage while retracting his spear was ultimately just a fortunate happenstance for Brennus. In truth, he simply wanted to free his weapon of the obstruction, and did so, withdrawing the point so it hovered between the two of them. Now should the man remain stationary he was just in time for his backstep to lead into another sudden forward gathering step. The point of his spear was placed nicely for another gut shot, so he thrust his back hand forward to shoot the shaft through his lead hand, with the intention of putting six inches of steel into the man’s lower abdomen. If the man instead moved towards him, he could thrust from where he was standing after his backstep with the exact same result. He aimed a couple inches off to the man’s right side in either case, as he was striking from his own right.