Fair enough, I'll test and see. Given the AI's improved use of the tech tree, I can't rule out that I've only ever attacked them with defense bonuses. I have found full health knights only take them down to 2 hp.
The HP number of a unit doesn't dictate how much damage it does. A 10HP and 15HP Knight both do the same amount of damage.
What matters is the attack value, and it (and Defense) get reduced equal to the proportion of current HP/max HP. So a Knight at 5HP does half the damage of a full health Knight (10HP).