I haven't given up on my theory. So I have done some searching. Darkrai is a dark-type pokemon who does in fact control nightmares. Dark type pokemon are not the weakness of time. Say that Darkrai has tried to stop them from going through time, he has to physically attack the tube they travel through. Saying that he manages to do so and causes the turbulence, he would evaporate on the spot despite the power he has. However, if Darkrai were really not wanting to risk death, he would have created a nightmare that they live indefinitely within. So saying that he attacked it could be a reason, but he would die upon contact unless he controlled time too. Darkrai could use his powers to sustain himself for a while to try and destroy the time warp, but he wouldn't have the strength to destroy it. But Darkrai aside, you remember when Dusknior said "The Dimensional Scream was gifted only to pokemon" right? He was human as you learned when you were pulled to the future, so if Dusknoir were correct and only pokemon could have that gift, could it be that this human in specific was maybe starting to turn into a pokemon? Or was it that maybe this human was gifted the dimensional scream by accident? Anyways, it could also be that he was human but turned pokemon during the time travel because time resumed and the time warp itself had no help to his transformation, and the turbulence had no help either. But hey, maybe I am wrong and Nintendo had a better reason. But that's just a theory, A GAME THEORY. Thanks for reading.