I have more time now and I may have been a bit too harsh earlier, so I'll explain how I'm calculating odds here.
If scum knew only 6 existing roles (JoeBob would know 8 as scum, but suppose he only knew 6), then this leaves 6 additional games, 3 of which are taken. The odds of asking about one game and it being in use is 1/2. If that game is in use, the odds of asking about a second game and it being in use is 2/5, since they are left with 5 games, 3 of which are not is use. So the odds that scum would fail to get a fakeclaim in those circumstances are (1/2)*(2/5) = 1/5. Conversely, the odds are 4/5 that scum would correctly identify a game they could use to fakeclaim.
The odds that the game with a town drunk is an available fakeclaim would be low, though, and very convenient for scum. So I don't think JoeBob's claim is fake.