Question about the fight between Asajj + 2 night sisters vs Dooku

I was recently rewatching clone wars, and it dawned on me that asajj came in and got extremely close to dooku and wasn’t able to be sensed by him AT ALL.

He didn’t react at all until obviously the dart physically penetrated him, so why could they not have just sneaked up closer and gutted him with a lightsaber?

Understandably, Dooku couldn’t have died due to him dying in episode 3 which is chonologically later in the time line, so is there a lore-accurate explanation, or is it just for the plot that they did this?

I’m doubting Dooku could have sensed them if they got closer, he didn’t know anything until he was stabbed with the dart.

Thoughts?