Not possible on console or PC

He might mean a 40 player mode that meets his standards. Maybe the bandwidth constraints of your typical online multiplayer game would cause them to have a downgrade the graphics quality and vehicle physics system in order to enable a 40 player mode.

They could be using Google's servers on the Stadia version, meaning all the players have a super high LAN like bandwidth connection to each other, eliminating the need to have to downgrade the 40 player game mode because of bandwidth constraints.

Dedicated racing games like GRID usually have vehicle physics systems that are way more advanced and demanding than you find in other types of games, so maybe having 40 vehicles using that physics systems is very resource and bandwidth heavy.

/r/Stadia Thread Parent