no, he is choosing to see something that is not implied. The Slayer is so ferocious in his attacks that even his misses damage your physical/Luck/Mystical defenses.
I don't know how you get slapstick out of that unless you choose to see it that way.
Please look back a the post you quoted, as there I was answering someone who was advocating a particular narrative form as the reason I should accept the slayer ability. I called it slapstick, because, in our example combat, his suggestions where slapstick for me.
As you brought it up, can I narrate the reaper ability?
Yes. To take our slayer fighter vs deathknight example, I'd do so after his second miss (as I noted the fighter misses three time in a row, the last hit killing the deathknight):
"(fighter-attack roll-miss)As you swing you already know you still didn't smite the deathknight, the powerfull swing once again going wide. But you've almost got him, his dark power is fading and his ghostly eyes are dimming, if you can just weather his next assault, it is over, you both know.
He comes at you again (deathknight-attack roll-miss), charging forward with a terrible stab, but it's not enough to take you down. You sidestep the attack and (fighter-attack roll-miss-3 damage take deathknight to zero), even though you can't muster a proper attack, his weakened state allowed you to position your blade such, that he went streight into it on his charge"
Yes, the ability can be explained and narrated, but it is jarring, because it really needs to be narrated on a case-by-case basis, it needs to be justified in the narrative, over and over again, far behind other, more consistent abilities. And thus I take issues with it.