When did Pierre Plantard mention Arsène Lupin?
Or indirectly make reference to Arsène Lupin?

Philippe de Chérisey stated he believed that the “treasure of Rennes-le-Château” was to be found in the fictional novel by Maurice Leblanc, “La Comtesse de Cagliostro” (1924).

Philippe de Chérisey was a very poor romantic. That can hardly be called a good reference!

But there are people who treasure his Large Parchment!

Even more than that, such people claim the Large Parchment is authentic and claim Philippe de Chérisey did not fake it !

Even when they are told the source of the text – and the mistakes in the Latin Script!

Denialist is a term used in psychology, describing an individual who actively rejects reality as a way to avoid acknowledging an uncomfortable truth. It's a defence mechanism where individuals reject reality, even when faced with substantial evidence, to maintain a more comfortable internal state.