It was a line that Magnus has already played once or twice in tournament. Hans wondered how Magnus could surprise him. So Hans studied various lines rarely played by Magnus including this one. This was very clear in Hans interview.
> In fact, Magnus has only played 4.g3 twice previously (both before 2010), and the position after Hans castled on move four had never been seen in any of Magnus’ games. Hans in a later interview commented that Magnus had previously played the opening against Wesley So in the 2018 London Chess Classic,24 but there is no such game on record.25 Magnus did play a g3 Nimzo-Indian against Wesley So in a rapid game in Kolkota in 2019, but the move order and emerging position in that game had no similarities to the game against Hans. Hans’s 9...cxd4 had only been played once previously, in a June 2022 Titled Tuesday game between Rasmus Svane and Stelios Halkias.