What does the term "diagnosis of exclusion" mean?

Not something else: It means that there is no test that can diagnose the condition, but there are tests to rule out other conditions that might be similar. If all those tests are negative, you might have the one condition that can't be tested for. When all other possible diagnoses have been excluded, you have the remaining diagnosis, hence the "diagnosis of exclusion".

Diagnosis : It is a diagnosis of a medical condition reached by a process of elimination, which may be necessary if presence cannot be established with complete confidence from history, examination or testing.