I understand what he is saying, which is ‘it seems strange to jump to concluding someone is hateful when their track record shows otherwise’.
However - people are constantly telling us ‘you agree with this hateful person so you are clearly wrong’.
Both can be answered, of course, by ‘water is wet’ but sadly I don’t think looking at who expresses what view on this subject gets us very far.
I wish I could agree with him that anyone coming to this cold will start from a position that Rowling probably has thought about the issue. Sadly, I think many younger people come from it from a ‘older people, no matter how well meaning, are wrong on a number of diversity issues’ angle. Whilst they may not therefore direct hate at JKR, they nevertheless are disposed to the thought that she may be wrong.