There is no contradiction. There are at least two situations that this rule covers which explain why two different calls can be made to restart play.
First, there may be no thrower, in which case ‘Disc in’ rather than ‘Stalling’ would obviously be the correct call.
Second, the defender restarting play may not wish to initiate a stall count but instead run to cover a free player elsewhere; again, they may call ‘Disc in’ instead.
So the rules do not contradict themselves, even if the word ‘stalling’ is required after the ‘disc in’ – clearly it is necessary to be able to call one or the other as required.
Benji Heywood