BPMNDI.xsd specifies attributeFormDefault="unqualified".
Yet many classes in camunda-bpm-model project use namespace for attributes.

All classes under package: org.camunda.bpm.model.bpmn.impl.instance.bpmndi

Here is an example from class:
org.camunda.bpm.model.bpmn.impl.instance.bpmndi.BpmnEdgeImpl :

bpmnElementAttribute = typeBuilder.stringAttribute(BPMNDI_ATTRIBUTE_BPMN_ELEMENT)
.namespace(BPMNDI_NS)
.qNameAttributeReference(BaseElement.class)
.build();
sourceElementAttribute = typeBuilder.stringAttribute(BPMNDI_ATTRIBUTE_SOURCE_ELEMENT)
.namespace(BPMNDI_NS)
.qNameAttributeReference(DiagramElement.class)
.build();