Exactly. So if we want to be pedants, earth or Earth would be fine here. Even if he's in a building, that building is still on the soil. Even if he's flying in an airplane, that airplane is supported by air, which it itself supported by earth. The only way you can't be "on earth" is if you're in space and/or in freefall. And Musk is too much of a coward to climb aboard one of his own rockets.