The argument from persecution is a fallacy that asserts that if your ideas provoke others to vilify or suppress you, then you must be right. Note: this fallacy is better known as the Galileo gambit. However, in this case, flat-Earthers themselves are the ones defaming Galileo and his ideas.
Flat-Earthers commit the fallacy when they show off that their ideas are being mocked, censored, or deleted. They get such the treatment not because they are right. But because by believing in a flat Earth, they have to accuse a lot of people of being dishonest and part of a global conspiracy.