They clearly refer to quarter-turn metric: “Finally, in 2014, it was shown that any valid cube can be optimally solved with at most 26 moves in the quarter-turn

They clearly refer to quarter-turn metric: “Finally, in 2014, it was shown that any valid cube can be optimally solved with at most 26 moves in the quarter-turn metric, or 20 moves in the half-turn metric [26, 25]. The quarter-turn metric treats 180 degree rotations as two moves, whereas the half-turn metric treats 180 degree rotations as one move. For the remainder of this paper we will be using the quarter-turn metric.”