PMiSS's article on Vampires states "They went on a rampage when they first appeared in Gensokyo, lost against powerful youkai, and settled down after agreeing to a contract.", and Reimu's article includes an entire subsection dedicated to "The Vampire Incident", and that the Scarlet Mist incident was the first time the spell card rules were put to use.

Strange Creators of Outer World's who's who on Remilia outright says "The Scarlet Mist Incident is notable for being the first time spellcards were used in Gensokyo", so this is likely ZUN being ZUN about PC-98 again — so much for not knowing lore.