BPMN21 — Derivation of categorizedFlowElements is only given by visual nesting

The specification says:

categorizedFlowElements:FlowElement [0..*] - The FlowElements attribute identifies all of the elements (e.g., Events, Activities, Gateways, and Artifacts) that are within the boundaries of the Group.

This attribute is derived and the elements are found by looking at the diagram and determining, which elements are within the boundaries of the dashed box representing the Group. As far as I know, this is the only place in the specification, where the location of an element in the diagram is relevant for the semantics. I think there should be a more formal way.
Further down the specification says:

Groups are one way in which Categories of objects can be visually displayed on the diagram.

If the only way to define a group is to show it in the diagram (as the first sentence implies), this sentence is wrong.