I'm not doubting your opinion of this bloke, I just want to mention a party trick a person with a good memory can do with chess.
If you're playing against more than one, and you arrange them the right way, you play alternately as the white or the black for each opponent.
Then in pairs you play the move your opponent played on the board of the opposite opponent. Basically you just mirror the moves so that each pair of players are playing each other. All you do is remember the moves.
There is no way I could do this, and I'm not saying Magnus is, but Derren Brown did it once to demonstrate the concept on his show, and every time I've seen a player play a bunch of others I've thought of Derren's show.
Also it seems to me that 'smart' is a bit like 'beautiful'. It is quite subjective and relies heavily on the context to have any meaning. He is clearly game smart, but could he sell ice to an Inuit?