Uh, they might wanna change the Steam description, which as of right now is:
The Day Before offers players a uniquely reimagined journey into post-apocalyptic open-world MMO survival set in the present day on the US East Coast following a deadly pandemic.
But also right above the categories on the right hand side where the store page tells you if it's single player/online pvp/online co-op, is the category "MMO"