Notably, books are exempt from this according to the article.
As for low rating "I received this book in exchange for an honest review", I've written plenty. As have most of the people I know who legitimately post ARC reviews on a regular basis based on books from places like Netgalley and Edelweiss. I've one starred books that Penguin sent me, and they kept on sending them.
I don't accept books directly from authors for review though (I used to, stopped years ago), nor do most of the serious review hobbyists. I one starred one of those once, and got threatened with criminal proceedings (obviously, that came to nothing
I suspect having a direct connection to the author would provide more incentive to inflate ratings.
Remember these are the consumer reviews being talked about here. Editorial reviews (i.e. commercial, or published in trad press) are only allowed in the separate space set aside for them on Amazon. The 0.38 positive bias can likely be explained simply by "most people don't request a review copy of a book they are predisposed to dislike" though that would take another study to prove.