A lot of the people who are now making noises about 'toxic Twitter' and what an awful place it is for authors are those who have gleefully led or participated in pile ons in the past, and appear only to be upset about these things now because they're directed at them and their friends. (Cat Valente in your link being the prime example!) There are also several people implying that the toxicity is solely coming from readers and the easy access to authors that they get via social media, whereas most of the worst instances of Twitter pile ons that I can think of were instigated and led by professionally published authors, and other promininent figures in the SFF and YA publishing communities.

To be fair, I am not reading every single reply directed at those who apologised (or 'apologised'), and no doubt there are some people who've leapt into the situation in order to shit-stir or settle their own grudges, but asking for accountability is not bullying.

I genuinely believe a lot of people involved in the mobbing of Isabel Fall feel that this sort of behaviour is acceptable as long as it's directed at the 'right' target, and if Fall hadn't outed herself as trans (in much the same way as Becky Albertalli felt pressured into outing herself as bi, etc, etc) they would continue to feel entirely justified in their behaviour. And many — rather than offering an unqualified apology — seem to be self-pityingly angling for absolution. They want their friends and community to tell them that they were right to have reacted in the way they did to the title of Fall's short story, and that their motives were pure.