If this kind of thing manifests more broadly, I think this can be a cool benefit of Stadia versions, re: providing scale experiences that other platforms can't offer as easily. The specific 40 number doesn't sound particularly impressive, but that idea is worth pushing.
I have my doubts about this being "real" in its current state. In the sense of the game engine needs to plow through all the data if they're truly expanding the scope, and a lot of racing titles out there are already a stone's throw from this on previous-gen, local-only hardware. For just plain engineering/feasibility, racing titles simplify the simulation outside of the player's immediate vehicle and surroundings, so it's easy to imagine this being a more superficial gimmick. But it's still neat, and one of the more feasible/fascinating pitches I've seen for Stadia as a platform.