I also thought p.21 was refering to the attacker having the option of enfourcing the Flag result or not when I first read it. When I went back through it though it really looks like it is stating that if the unit isn't destroyed then he should decide, by checking to see if there are Flag results, whether or not the unit is pushed back. The section on Flag results clearly states the the defender must fall back.
But for the sake of argument lets say that someone from FFG says it is up to the attacker to decide, if the defender is Stalwart it would seem that the attacker would want to push them back anyway to prevent counter attacks.