In the case that it doesn't fit because if it did fit it still wouldn't work then I'm not going to try and make it fit. Yeah so must be an error on Crucial's part. I guess I can contact them to correct it.
Interesting. When I go check the memory compatibility on their web site for Mac Pro 2013, the results page has a different look and layout than your screen shot, plus it only shows DDR3.
I'm on desktop. Maybe their mobile website is broken.