Anger is a combination of a feelings mixed with a judgment or blame of others
Underlying anger is one or a combination of emotions such as fear, disappointment, frustration, resentment.
Is this an assumption about NVC or a general assumption? In other words, is this an assumption that is necessary to practice NVC?
Maybe the core assumption is "A feeling vocabulary is important to express your experience more fully." There is an assumption around the need to increase your awareness of feelings and needs.
When we express any emotions without our needs we are considered to be less competent than when we use needs and requests as well. The opportunity is to measure the difference in perceived communication skill when people use feelings or NVC.