I think parent is saying that the progressive Twitter mob has been labeling and attacking people who support merit-based admissions policies that ignore race as

I think parent is saying that the progressive Twitter mob has been labeling and attacking people who support merit-based admissions policies that ignore race as racists for many years already. This cancellation is not “different” as the author says, they are only now becoming aware of the pervasiveness and wrongness of this type of thinking since it happened to someone they care about and not someone they didn’t.