I recall the DoomGuy/Erdrick/Ryu Leak etc.
had explanations of their inclusions and release dates for the characters.
That alone told me it was fake. That was way too much details for a random leak, plus the dates seemed unrealistic. Mementos being the stage isn't going to change that since it was one of the most common predictions for joker. All I kept hearing was Casino or Mementos. Before the leak even happened as well.
Off-topic: Memento is a fantastic film. Have to mention that since cause I love the film :P