The Grinch Leak was a massive case of "too good to be true". It came right as pessimism spread throughout the Smash fandom that Ken and Incineroar would be the only remaining base roster newcomers, and then this rumor comes around that was basically designed to make people excited about the game. It predicted that there would be seven additional newcomers, those being major fan favorites like Geno, Banjo & Kazooie, and Isaac, two obvious Echo inclusions in Ken and Shadow, the Chorus Kids, who were the final remaining characters implicated in the infamous Gematsu Leak who hadn't been added to the game yet and were thus heavily implied to be scrapped from Smash 4 during development, and finally Mach Rider, who fit in the role of the "retro surprise pick" that each game has had since Melee with Melee having the Ice Climbers and Mr. Game & Watch, Brawl having Pit and R.O.B., and Smash 4 having Duck Hunt. The leaked image of the full roster was even placed alongside what appeared to be marketing material for the then upcoming movie The Grinch, which only added legitimacy points to the rumor. I think something else could come along to strike the same level of impact that the Grinch Leak had in October of 2018, but I don't know.
