Whilst I found it very interesting I'm perplexed to how rats were included in the study, rats or any rodents come to that have to chew to keep their teeth down as they continue to grow. The reasons both species chew are entirely different.
I can see why fearful dogs would improve with chewing as its self soothing and dogs are then better placed to learn and remember tasks.
Rats are also excellent at learning and remembering, a lot of the understanding of operant conditioning used in modern dog training, is based on Skinner boxes/the behaviour of rats.