It's not a chance, and yes it is terrible wording.
A better wording would be : "Opponent Health for Deathblow", which I use in my mods.
I believe Dmitriy uses "Opponent Health Threshold for Deathblow" in his mod, to really spell it out.
Death Blow is double damage, simple as that.
Death Blow doesn't operate by giving you a chance to trigger it, instead, when the opponent's health falls below a certain threshold, all your hits inflict Death Blow (double damage) on that opponent.
So the % number you're looking at is the threshold of hitpoints the enemy has to fall to before Death Blow starts triggering.