In his description of the test setup, he says that he powers the Game Boy with the DC jack input, so I assume he's measuring the power consumption of the entire

In his description of the test setup, he says that he powers the Game Boy with the DC jack input, so I assume he's measuring the power consumption of the entire machine, but I might be misunderstanding that. Either way I'd expect the Tetris cartridge on itself to consume more power as well, as during the busy loop the game is constantly reading the next instruction, whereas games which halt don't access the cartridge at all until an interrupt happens.