I agree, this sort of "single player" interest is definitely a feature of ArM. Particularly as all the players can muck around, thinking about spell and item design, and then gain benefit from having done this when they do meet for an in-play session. This sort of "preparation" is not just a chore left for the Storyguide, and it can continue even when the player isn't involved in an active saga. It is the origin of much activity on the forum too.
It is very similar to the way that there is enjoyment to be had in designing Battletech mechs or Traveller spaceships.
As the Storyguide for my current saga, I think the 'hardcore fan' definition comes in when I find myself doing research on what most people (even most RPG players) would consider unnecessarily detailed reading as background for the saga. For example, one of my most recent readings is a journal article from the British Agricultural History Society, "Non-Manorialism in Medieval Cornwall" so as to get a better sense of how medieval Cornwall worked given the placement of a new PC covenant there.
In a traditional D&D campaign, I suspect most GMs would just pull the details for the "County of Dendraxillus" out of thin air and the players would just nod and move on. Not Ars Magica players!