my guess is: Tumblr fandom used the tag to mean various things; the discord names itself after the Tumblr tag; people started to tag for the discord on AO3; someone on Ao3 realized the discard name and the general meaning were getting mixed up, and created the disambigged tag (I can't see why else it would be disambigged w the fandom - a tag meaning creators don't read or don't repost would be treated as a panfandom concept); the disambigged tag was not clear enough either, and now it is also being used for both things.