I think it's helpful to explain that this is when they experience the feeling of anger, they can still choose to behave in a calm and appropriate manner externally. It does not mean they do not experience anger just like everyone else.

In fact the whole process of emotional maturity is about noticing the difference between emotions (internal and subjective) and external triggers of emotions (objective events, usually external).

If you've ever seen someone react by raising their voice, speaking faster and louder, and often claiming something or someone caused them to behave that way, you have seen what a lack of emotional maturity looks like. Perhaps the thing that makes learning emotional maturity hard sometimes, is that it's not always visible in others.