It's a very smart way to deliver his message, and to keep people on his side by establishing some familiarity and common ground with his audience - he stabilizes the waves with it. It's being humble and to still give someone that much credit when said person is incapable of any such nuance is a great PR spin proofing it. Anyone seeking out his statement via any click baiting from the POV from a JK moan, random right wing tweet or whatever is immediately put at ease by him starting with that. It's quite brilliant really as of course he would have gotten other roles and being nice to her just exposes her more. If she were to reply to it, she couldn't make his words menacing as he has nothing but kindness, fairness and self-awareness. He makes himself the relatable one for the general public, and the HP fans are only having their favourite books/films ruined by her and not him.