You are right, the notification icon looks like a bowl and the picture shows a singing bowl, too.
In the tradition of Thich Nhat Hanh bell is meant more generic - a mindfulness bell can be the sound of a temple bell or a church bell or a tibetan bowl or it can even be a red traffic light. To read more about the bell of mindfulness in that tradition please read this.
And, as of this article singing bowls are a type of bell.
Therefore I decided to stick with the bell.
However, MindBell uses the term ‘the bell rings’ instead of saying ‘to invite the bell’ as it’s usual within that tradition.