A conspiracy is "an unlawful, harmful, or evil plan formulated in secret by two or more persons; plot."
A conspiracy is not inherently fictional or nonfictional, it is a plan secretly devised by multiple people for an often harmful purpose.
In the same way books can be fictional and nonfictional, conspiracies can be fictional and nonfictional.
Fictional book: Harry Potter
Nonfictional book: The Diary of Anne Frank
Fictional conspiracy: Vaccines contain microchips - there is no evidence supporting this claim.
Nonfictional conspiracy: The Fake Electors plot - a documented and testified to attempt to steal the 2020 US presidency by Taco and friends by lying about the US election results
Nonfiction accounts are supported by verifiable, corroborated evidence.and has no rational basis for dispute
Why YSK: I see many posts and comments conflating the word "conspiracy" with "fictional scheme", which is not at all what the word "conspiracy" means.
the misunderstanding could be rooted in "conspiracy theory", which is the often unproven suspicion there could be a conspiracy. those are often entertaining mysteries, but some ppl take them for granted, because they would enjoy them beeing true. slippery slope