BPMN21 — Subclasses of Interaction Node (Task vs. Activity)

According to the Message Flow Class Diagram (Figure 9.14), Interaction Node has four subclasses: Participant, ConversationNode, Task and Event.

However, according to page 124:
Of the types of InteractionNode, only Pools/Participants, Activities, and
Events can be the source of a Message Flow.

The question now is, whether Task or Activity should be the subclass of Interaction Node.

Suggestion: The Message Flow Connection Rules on page 44 specify that Message Flows can also connect to Sub-Processes. Therefore, I think it is necessary to define that Activity is a subclass of InteractionNode.