We generally don't cue external rotation at the top of the snatch directly, although that is more or less the result. We do emphasize the drop underneath the bar coupled with turnover of the wrist at the top and a shrug up.
I think the confusion exists because the motion of the shoulder joint is fairly complex. You are in internal rotation at the hang. As the bar is racked overhead, you are arguably in external rotation, although you did not perform that external rotation in quite the same way you would if you just stayed in the hang. Do people argue that you should be in internal rotation when you rack the snatch overhead? I have not heard that before. It would be very difficult to do while holding a barbell.