While giving evidence at the trial of Stephen Ward, Rice-Davies made the
quip for which she is most remembered. When the prosecuting
counsel pointed out that Lord
Astor denied having an affair or having even met her, she replied, "Well, he would, wouldn't he?" (from Wikipedia).
Hence, "MRD applies" means "he would say that, wouldn't he". Usually used to devalue an opinion by drawing attention to the other-than-impartial situation of the person
voicing it.