Just a few observations:
1) There is of course the question of weather the 67 slide is critically sharp.
2) The 67 slide is taken with a Pentax 67 with a 90/2.8 lens and it is very well possible that it is less sharp than your Mamiya 7. The tests I have seen on the Mamya lenses were generally excellent while the Pentax is at best middle of the road.
3) The Pentax 67 has issues with vibration from both mirror and shutter, but I controlled this as well I could.
4) The image you see is a very small crop at actual pixels. The original image is about 7000x8500 pixels the crop is about 1000x1000 pixels. Grain is clearly visible, so the scanner has resolved that.
5) The image is shot on Provia not Velvia (Velvia has better grain in my experience)
6) Regarding colors IŽd suggest that you are slightly in error. Colors are created both in capture (and determined by the filters in the bayer array) and in postprocessing, where you can essentially achieve any color.
7) The engineers at Fuji developed a color rendition for Velvia which is much liked by many photographers, but far from all. Joseph Holmes does not like Velvia at all, for instance. I'm pretty sure that digital capture can be processed into Velvia like color.
I could unfortunately not find the link to your post, I sure would like to check.
These images were not intended as test images, but I happened to have shot the same subject with the two different systems. A side by side comparison shooting the same subject at the same time may have been less error prone.
A side note, I have noticed that some photographers don't like digital, I have respect for that. It seems that a majority of photographers prefer digital and I also have respect for that.
Read the post I linked above of my comparison ZD vs. Velvia 50 in Mamiya 7. I am rather certain the ZD at low ISO is more sharp than the Sony. Thus, proper scanned on a drum your 6x7 slide is destined to beat the Sony in resolution, colors etc hands down. Care to make a bet? It serious would be interested see that slide drum scanned, if original is sharp.
I must honest state that not even my 35mm slides from Minolta Dimage SE were lack of detail as that scan of 6x7.
Above per my honest experience. Thus I shoot more film in addition to digital; now also 4x5.