The study is indeed very interesting, but I would like to know if the diets of the dogs were comparable. There is no way to know, based on the paper, but if the subjects were matched with respect to other characteristics, I would expect that to be stated in the abstract. I am guessing that the subjects were not matched in other aspects.
I can envision that more of the intact dogs that are involved in showing and breeding are less likely to be fed a low quality kibble and less likely to be given low quality treats with possibly carcinogenic preservatives such as BHA/BHT.
I seems to me that the difference between the dogs of the early alter/ late alter / intact trait might not be the only difference. This is frequently a difficulty with statistics.
Disclaimer: When in college, I wrote a class paper titled, "How to Lie with Computerized Statistics", inspired by the book, "How to Lie with Statistics" by Darrell Huff.