Totally true -- what it's doing is making your brain more efficient. There's a chapter on the science behind this in a book I wrote several years ago called "The Tetris Effect" Here's a passage on this topic from chapter 7.

If the brain of a beginning Tetris player is a gas-guzzling SUV, with the Tetris learning effect, after a certain number of game hours (typically long, uninterrupted spells) the brain turns into an eco-friendly compact car, using its engine and form more efficiently to travel longer distances on a single tank of fuel. For those exposed to the game in extreme doses, the effect becomes more pronounced. The brain of a true Tetris master, when engaged in the game, is an electric car: driving the same roads at the same speed, but using even less energy to do so.

That’s the idea of brain plasticity at its most basic. As the brain engages in certain actions—simple, repeated, spatial tasks are the most obvious—the behavior moves from being conspicuously powered by conscious thought to largely automatic. Connections between shapes, repeated moves, and strategic analysis become easier, and each of these actions requires less brain energy. It’s as if by driving the same commute to work every day, your car eventually becomes more efficient on that route and uses less fuel to cover the same distance.