This is extremely common in high level chess. Every chess player will study their opponents history and recent play patterns and practice against that.
Hans claimed he studied against Magnus' opening because Magnus had played it a few months ago. It turns out Magnus has never used that opening in a recorded game. The dialog has now changed to "well by move 20 the board state became identical to a previous Magnus game" but Hans didn't say he spotted the similarities at move 20, he said he studied that specific opening.