You should've spent more than two seconds on it, for there are actually at least three conflicting theories of its origin.
https://www.bbc.com/news/blogs-magazine-monitor-34298659
https://www.etymonline.com/word/fan
http://medkult.upmedia.cz/2016/08/0...-movement-brief-history-of-the-first-fandoms/
So Gerrold apparently favored William Henry Nugent's derivation, and I agree with him. Since the evidence shows that aficionados were already called "the fancy" before the word "fan" caught on, it does seem more credible, despite the tendency of dictionaries to favor the "fanatic" derivation without listing the alternatives.
Etymology, as I said, is not an exact science. There is no magic crystal ball to give us absolute certainty about the origins of words; we just formulate hypotheses based on incomplete evidence (because there is no written record of the origins of spoken language) and what seem to be reasonable deductions. As with any science, those deductions are presented merely as plausible hypotheses pending further evidence, not indisputable dogma.