Just so you know, this isnt a mate in 6, its a mate in 3 after ...gxh5 (as far as i can tell). You choose to make the puzzle 6 moves before Carlsen's opponent was mated.
Magnus Carlsen-Tricky Mate In 6

mate in x number of moves is a chess expression - it means that a forced mate is made, in maximum that number of moves.

I'm not sure I follow you, as these are the final six moves of the game.
What Tactchess means is that the mate in 6 wasn't forced since Kh8 or something else would have prevented mate I think. Yes, these are the last 6 moves of Carlsen's game against Hans Harestad but the mate was only forced after gxh5.
Bonus: Can you name his opponent?