A flail isn’t a STR weapon. It’s actually closer to a DEX weapon. Flails are much more difficult weapons to use, because you can’t stop them easily once they’re swinging, so you need to be skilled with them to keep from braining yourself.

Hammerhead grabbing the chain there wouldn’t STOP the flails forward motion. He’d need to grab the head of the flail itself, or MUCH closer to stop it. With that much chain left, the flail would just continue on its original course. Best he could have done would be to alter that course slightly, angle it up or down. But it would still have kept going around his body to the right.