I think what he was trying to say is that you're much more mentally engaged when the medium challenges you to actively participate (like using imagination to visualise the book you're reading, or trying to solve a problem in game) instead of just absorbing information passively.
This is likely it. There's always some component of thought in a game, even if it is highly repetitive (though there's a point where the player being nothing more than the controller of which thing is clicked in what order approaches the level of passivity inherently connected to television. This is, I think, one of the reasons why we as a community and gamers more broadly favor games which either mimic a book or pen&paper RPG (DF, Aurora, various other sim/strategy/RP/roguelike games) in terms of allowing players to fill in the gaps with their imagination and build stories, or games which pit them against other players or overwhelming odds in situations which both encourage and require very fast tactical and strategic planning and execution.
Of course literature (small-l) is still often better when it comes to fueling imagination, but that doesn't mean that other media can't encourage thought. I can see where SG was coming from when I consider that of the people I know, they are almost all divisible either into the category of "gamers", "non-gamers who watch TV and play Call of Duty in between drinking binges", or "workers who spend their leisure time watching TV". The only reason I can't count the number of people I know who read for pleasure on my fingers is because of my family full of Arch-Liberal artistic types.