It is the intersection of two factors. First, Lemmy is a niche social media platform. The only people who know about it, let alone use it, are extremely online to begin with. Second, most Liberals are genuine idealists who believe rhetoric is the primary arena of struggle. They believe that the reason Liberal Democracy is crumbling all around the world because people are being misinformed (not because of the underlying economic failures).
Combine these two factors and you end up with the type of radlib fundamentalists we see on Lemmy, who believe rhetoric on social media is the primary arena of struggle, and that people like us are either a social contagion spreading among ignorant dupes, or malicious foreign state operatives.