Diffusion is based on depth of panels. 7.5 inch depth is limited down to 1100 Hz, thinner will be limited to higher frequencies. Design shown provides very little opportunity for absorption. Sorry laws of physics apply here.
I use 10 GIK 244 2ft x 4ft panels (found here at Audio Circle) in my 8ft x 13ft x 21ft room. 2 full range version on the front and back walls, 1 on each side wall. 4 bass trap versions straddling the front corners. These are absorption panels. Note that I also have 3 tall bookcases on the side walls used as "casual" diffusers. Not all absorption panels are created equal. Check the specifications (most don't even have them). IMO GIK knows what they're doing. These panels are highly effective. My six full range panels made a tremendous difference in a restaurant that had huge reverb issues and a significant difference in a friend's rather awkward room, frankly the 10 panels I own make less of an improvement in my "ideal" room but was more of an audiophile mania move.