Yes you are right, some of the names are clashing there. I will fire a Jira issue to fix them as appropriate.
One way to get things clear is to say that all the classes inside the org.jbpm.bpmn2.xml package are internal to the parser and should not be used when you are developing a process, because they are internal to the framework.
For domain specific activities you should use WorkItemHandlers that are the generic way of interacting with external entities from the process perspective. If the activity is done by a Human or a System doesn't care for the engine. We provide some HumanTask related work Item handlers (that's where the name clashes) but using any IDE you can take a look at the source and realize that they all extends WorkItemHandler, as you notice.
Greetings.