It was hinted at in the article, but the publisher sells the book to Amazon for less than the cover price. Of course, I have no idea if Amazon's purchase price is a percentage of the cover. But if it's not (and that's what I would have negotiated) then changing the cover price won't affect the revenue to the publisher.
Retailer's purchase price is almost always a percentage of the cover price.
Then I guess it's up to the publisher to do the math and see what works out to more profit in the end: keeping prices the same w/ less volume, or dropping the price and (hopefully) increasing the volumn.