Jon's upfront fighting style and Moster's bestial fighting style had Graham and his men fooled.
In most cases, shadow-type fighters are good at ambushes just like the other two knights in the group. They tend to be quite lacking when it comes to a frontal fight.
Shadow and darkness users mainly cultivate burst-type attacks to make the best use of their concealing and unpredictable abilities.
That in turn makes them bad in long-drawn fights.
When Jon showed his ability to fight long-drawn fights, the enemy had already assumed him to not be any type of elemental cultivator, at least not a powerful one, and most definitely not a shadow user. So, when Jon used his shadow dive it shocked everyone.
As Jon got out of the shadow, he was already out of the encirclement and was looking at the back of the enemy that was about to launch an attack on Monster.