That anti-fanfic thread was actually a pretty good collection of dishonest argumentation tactics and logical fallacies.
Tautology, question-begging: If all fanfic were really “low-effort formulaic lowest-common-denominator writing,” this tweet would be a tautology, but since that’s the question under debate, merely asserting it is begging the question (aka assuming the conclusion).
False dilemma: One doesn’t have to choose between reading/writing fanfic and reading/writing “legitimate” fiction.
Distraction: Amazon’s crappy working conditions have nothing to do with fanfic. One might as well ask Benedict why she’s complaining about fanfic instead of being concerned with Amazon.
Appeal to authority: Benedict asserts that being a member of a minority group immunizes her from accusations of identity-based bias.