Senior White has an innate spacial ability, it might have also been applied to the weighted seal he put on Song Shuhang since he knows about the lotus world. I suppose it depends on how strong White is as a new rank 8. The rank 9/immortal(?) stone giant was able to seal space around Shuhang to prevent him immediately fleeing, but the lotus world itself was able to eventually overcome it. Also, like Senior North said, White's luck would probably kick in if he really wants Shuhang to stay sealed and Shuhang tries to escape.