A black player who wants to play the King's Indian, but encounters the London instead, often chooses a system with the move ...d6. For instance, the book "Fighting the anti-King's Indians" by Yelena Dembo advocates such a system. The pawn on d6 restricts white's bishop on f4, and common white plans such as putting a knight on e5. This system scores very well for black.
With the move order 2.Bf4 3.Nc3, white threatens to play 4.e4, transposing into a Pirc. The move Bf4 isn't common in that, but Be3 and Bg5 are and they are often followed by a later Bh6 by white, so this system can transpose into a reasonably normal Pirc. That's an opening black may not want to play.
So black plays ...d5 to prevent e4.
As a result, Nc3 is in a slightly strange square, but Black has been prevented from playing his very favourable ...d6 setup and now has to make the slightly unusual combination of ...d5 and ...g6 work somehow. This is a little explored situation and the players are on their own. The best player is likely to win.
That's what Carlsen wants, as he is the best player.