Dogs have spleens unless they undergo a spleenectomy.
Not sure what is meant about dogs not being able to do well on vegetarian diets. There would be many Yorkies in trouble if that were the case.
AAFCO is not perfect and there are two ways to get a diet approved. One is a nutrient analysis and the other is a feeding trial (done on the life stage that is represented on the bag/can). The latter may be preferred and is a good indicator of how dogs will do on the food.
Here is a study:
Cat Fanciers' Association: Role of Diet
that shows what can go wrong. Apparently rabbit isn't deficient in taurine, but the cats couldn't absorb it. And it looks like their vitamin e levels were low. My point being is that there is no clear answer here that this is the best diet...far from it. And this 10% one type of meat and 15% that type of meat...nto sure where that is coming from, but wonder where they came up with it at.