This is an unusually vivid example of the basic (and obvious) problem with identitarian deference (ID), which is that it can be wielded in defense of literally anything. Most critiques of ID content themselves with simply establishing this point, but if we accept it (as the Greer episode suggests we should), a second question presents itself: what sort of person is likely to abuse ID?
Psychologically, the answer strikes me as pretty straightforward. Martha Stout, in The Sociopath Next Door:
...there is an excellent reason for the sociopathic fondness for pity...the explanation is that good people will let pathetic individuals get by with murder, so to speak, and therefore any sociopath wishing to continue with his game, whatever it happens to be, should play repeatedly for none other than pity...
...pity from good people is carte blanche. When we pity, we are, at least for the moment, defenseless, and like so many of the other essentially positive human characteristics that bind us together...our emotional vulnerability when we pity is used against us by those who have no conscience. Most of us would agree that giving special dispensation to someone who is incapable of feeling guilt is a bad idea, but often, when an individual presents himself as pathetic, we do so nonetheless.Fundamentally, ID relies on the recognition of our shared humanity with the oppressed and our sympathy for their plight - and it's easy enough to see how sociopaths could abuse this. What Stout's work contributes here is the insight that sociopaths will indeed tend to do just that. Given the opportunity, they will try to abuse identitarian deference, because it gives them carte blanche to prey on the sympathy of others. This is a tendency confirmed both empirically and by our best theoretical understanding of how the psychology of sociopathy works.
ID's potential for abuse does not, itself, necessarily delegitimize it as a valuable norm or consideration. Greer, presumably, would plea that he made a good faith mistake, and that he invoked ID on that basis, rather than in a cynical attempt to defend convenient slander. Proponents of ID, meanwhile, may insist that it is on balance progressive and beneficial, even if it is occasionally abused. These are all completely legitimate positions as long as we bear in mind ID's vulnerability to appropriation by actual, literal sociopaths. Nothing about identitarian theory rules that out, and everything we know about psychology affirms it.