I think you are mistaken here. I’ve startled dogs mating before. In their attempt to flee from me, they force themselves to separate...all the while yipping and crying from the pain. No one or anything was bitten. They run away. I could see a dog fearful of discipline not attacking. THat is assuming this crazy story is true. I personally suspect is is not.
They don’t always lock; but this story suggests that the dog was still attached to the child.
They may have just begun the process or finished it too.