Yes, that is true. Dynatrace does only detect a remoting service when there is a server side detected as well. Because of different deployments it could be a local RMI call also.
So there are no details of RMI calls to a sort of "unmonitored hosts"? In my case, the RMI server is on an IBM iSeries, so I guess I will have no luck, because I'm monitoring only the client making the calls...