I am sorry... I do not pretend to be an expert in this area... but I would like to point out... I think the main difference between lenses of the same focal range (eg, prime lens of 300mm f2.8, 70-300mm at 300mm), was the resolution of the image that then lens could capture, in another word, some of the lens couldn't resolve the details of the overall pic, while others could. This could be caused by lots of variable, such as the design of aperture blades, the number of elements in the lens, the coating (or lack of it) on each element and the manufacturing and assembly of elements.
As to the phenomenon as to why (as claimed) a 300mm f2.8 lens produce more 'contrasty' images as compared to the 70-300mm when shot in 300mm... might easily mean that,
1) The confusion of image isolation to having greater contrast as compared to a lens with smaller aperture, whereby 300mm f2.8 shot wide open would have better subject isolation because all other parts are blurred out, while 70-300 having a smaller aperture, would mean more of the picture are in focus and not blurred out, so the image might came out abit 'messy'.
2) The manufacturing process is different between 300mm f2.8 and 70-300mm.
3) 300mm f2.8 might produce sharper image as compared to 70-300mm lens (there are many reason behind this claims... but it is by no means a claim that prime lens are sharper than zoom lens).
4) the 300mm lens might be design and build to resolve more details as compared to the 70-300mm lens thus mistaken to produce much better contrasty image... which might be a different thing.
5) The 70-300mm lens that Devilry used had bad lens haze on it...
Again, I am not an expert in this area, just wanting to voice my opinion