That’s right, but it’s just a simplification for the sake of rendering. The BpmnRenderer does not need to know what kind of event definitions are contained within the Event, but it only needs to know that there are more than one definition.
Nevertheless, bpmn:MutlipleEventDefinition is not a valid BPMN 2.0 type.
As far as I understand you want to be able to create events with multiple event definitions through the palette, the context pad and the replace menu. bpmn-js can already render all BPMN 2.0 elements so adding support for multiple events is a matter of adding the ability to create them. Note that there is no existing solution for maintaining multiple event definitions. bpmn-js-properties-panel only supports single event definitions.