Downloaded it and gave it a go, I'm 97% sure that it's just a single-player game faking multiplayer using bots. There's absolutely no latency/ping/rubberbanding AT ALL. ... That said, it's literally impossible for an online physics-based game to perform as well as an offline single-player one.

Is stumble guys really multiplayer?

Stumble Guys is a massively multiplayer party knockout game with up to 32 players online intention to struggle through levels round after round of escalating chaos, until one victor remains! If you fall, just start again and run. Join the endless running fun!

Do they put bots in Fall Guys?

Bots are user accounts often used by automated, user run scripts which are used primarily to accomplish repetitive tasks. Membership in this group automatically hides their edits on the Recent changes page default view. They can be displayed by clicking on "Show bots".