Is there a mistranslation of definition of ignorance?
There are 3 types of wisdom 


suta-maya panna: wisdom gained by listening to others
cinta-maya panna: intellectual, analytical understanding
bhavana-maya panna: wisdom based on direct knowledge or meditative(Vipassana) wisdom


Knowing here means the third type. When a monk preaches or when you read a sutta on your own you usually gain type 1 and type 2. Those are not strong enough to cut off craving. You need to see the four noble truths based on direct knowledge. In other words, through Vipassana meditation.