It's a bullshit conclusion from a crap study.
Their responses indicated that according to dog owners, raw meat was a healthier diet, but they dismissed those because (among other reasons) those owners seemed to take the dogs to the vets less, which is exactly the metric they used to ascertain that vegan dogs were healthier.
They made no effort to correct for the myriad of other factors that impact canine health (breed, age of dog etc etc) apart from, mysteriously, in the raw meat diet.
Which brings me to the point that its a self reporting questionnaire based study, and therefore tells us almost nothing about dogs, but a little bit about the owners of dogs (this is a bit of a worrying trend in domestic animal studies currently).
Who'd have thought that a study funded by ProVeg, published in a journal nobody has ever heard of (what on earth does their peer review process look like?) comes up with some very dubious conclusions?
Also, you might find, if you bothered to look that there are a few studies actually done on dogs that don't agree with these findings....