I think influence is importantly different from power in this context.

JK Rowling might have had some role in shaping society through her works and have some platform to speak because of her fame and fortune but no one was obliged to do what she said or take her seriously.

Everyone who wanted to had read her books and seen her films or could assiduously virtue signal by taking convoluted and public steps to consume them without paying Rowling any money. Unless you were actually a recipient of her charitable donations you had nothing to lose by shouting at her on the internet.

Whereas, if you wanted a career or any sort of position or role in the Labour Party Corbyn could ruin your life or at least dent your aspirations.

I don't think Rowling went about actively trying to have people expelled from Pottermore

I'm not surprised that people were a bit coyer about how they dealt with Corbyn.

There are other differences that played in to this but I think the fact that Corbyn could ruin your day in a way that Rowling couldn't or wouldn't is important.