Words also sometimes have different meanings in different places, and in different cultures. I’ve been a member of many other discussion boards over the years (not so many now that they’re an endangered species), and most of those boards had a few words or terms that seemed to have a unique meaning there, because it had evolved as part of the particular culture that developed on that board.
In this case, the SDMB-specific definition of “threadshitting” is something that most members of this board seem to understand, because it’s part of the unique culture of this place, and because it’s been part of the board’s particular vernacular for a long time, just like “Hi, Opal” or “conch.”
Did someone set out to give us a made-up definition for that word, 18 or more years ago? I kind of doubt it. More likely, the definition to which @Colibri pointed you was a codification of what was already being used.