I was hoping to get an official answer on this question since the wording isn't 100% clear on the ability. Have any of you seen anything official that would vouch for how to treat the ability or is it personal interpretation?
I think you might be reading into it too much. Sam can cancel any "Fight:" cards that a zombie plays during a fight with him. It doesn't matter if they allow the zombie to roll extra dice, or force Sam to re-roll his, or whatever. And, again, he is only able to cancel them when he rolls a 4+ to try to cancel it.
Any Zombie "Fight" cards used during a fight with a Hero are technically being used against that Hero.