bro that tech demo was a tech demo which had aspects of it turned into other games, notably Pikimin(as there were 128 marios) and the way platform worked influenced Super Mario Galaxy's development. It never became a full game in the typical sense
It was a publicly used codename for the next Mario platformer late in the GameCube's lifespan*. Miyamoto later said it was just experiments and concepts, but the few things he did say about it (Mario having a new partner, a new camera system) actually were in Super Mario Galaxy, the 3D Mario that was in development during the Super Mario 128 era.