Dec 27, 2018
This; https://m.fanfiction.net/s/4112682/1/The-Subspace-Emissary-s-Worlds-Conquest is the fanfic they are talking about. Currently 4102k+ words.
THIS; https://m.fanfiction.net/s/12968427/1/Super-Smash-Bros-Under-Fire-The-Great-Adventure-Saga is the Super Smash Bros fic I found that was even longer. Published June 13, 2018. Currently 7296k+ words. It isn’t the best but it IS longer.
Fanfic: Super Smash Bros Under Fire: The Great Adventure Saga Ch 1, Super Smash Brothers | FanFiction
Fanfic: The Subspace Emissary's Worlds Conquest Ch 1, Super Smash Brothers | FanFiction