A discipline being superfluous?
Superfluous usually means unnecessary. So the person is claiming that a discipline is unnecessary. Presumably, this is a claim that can be investigated further: does research in that discipline have any kind of external impact ? does it affect other related disciplines ? Is there at least a path from the questions being asked to some future impact ? 

In fact these are all good questions to ask about any discipline, whether it's deemed superfluous or not. And these are good questions to ask about one's own research in a discipline. 

p.s While this is off topic, there's a common feeling among CS folks that HCI is either fluffy or superfluous. All of these people are also forbidden from using a Kinect, any video game console, a tablet interface, or even a smartphone.