I don't have film of those guys and open stance is always possible. BUT for the current top men with the arm structures noted above, the preference is closed when on the backhand side toward the sideline, more neutral stances when closer to the center.
Because the body is less open at contact and the role of the front arm, so long as the back foot set up is good, cc is no prob. In fact prefered.
They are doing this in my opinion because they get more pop from the initial hip and shoulder rotation caused by increase turn in closed stance and the use of more front arm.