There's a difference between arguing that draining three council masters proves Traya can use that power to defeat legitimately high tier characters, and arguing Zannah's use of the power on a character with high tier willpower proves she can use it on other high tier willpower characters.
I'm not arguing Caedus can't overcome it, but I doubt he can do that while simultaneously fighting a duelist who has pressed Obi-Wan's defenses to their limit and a combatant who has injured and contended him with a terrain advantage. Grievous and Mara together are a bit more than a toddler with a lightsaber.