The man perhaps was overconfident in his footwork. The spear’s point would not simply miss his face because he was moving forward, for several factors.

Brennus had first thrust from range, the ground to cover was significant, and that stood against the man’s intentions. Brennus had also stepped backwards upon launching his own cut, carrying him at least as far backwards as his foe sought to move forward. Finally, a spear is more malleable than the man gave it credit. Brennus simply pulled the haft of his spear further back behind his body while making his cut, subsequently bringing the point to bear towards the man’s face, and him having taken no action to defend himself, he would suffer. Such was only human.

Furthermore, Brennus could lead into his follow-up. Upon drawing the haft back sufficiently to still cut the man’s face, his left leg drew level with his right, and as he stepped back with his right foot his spear would be level with the chained man’s throat. With the man rushing him, it wasn’t necessary to put much force behind a thrust as he was doing the work for him. So Brennus simply brought his right hand in line with his body again to thrust his spear forward a little towards the man's throat. The chained man would do the rest of his job for him, likely skewering himself on the point, and ending the fight before it really began.