Spinarak prevents the foe from retreating, that's how. If the Durant player doesn't run Switch, you can simply lock them until time runs out, then take a KO and win. The problem is that if the Durant player catchers something and you can't retreat back into Spinarak, they can resume milling. Anyway, I personally think Unown Cure is an awful tech for Chandeplume because first of all, it does nothing to hinder the damage of its ability, which is what you should be worried about. The statuses are really no problem, just retreat and you've done all Unown Cure can do, without putting a weak Pokemon on the best that they can KO for a prize in 1 turn.