I think it is a bit early to come to any conclusions based on Nikon JPEG samples, I would not be surprised if less-than-perfect technique was employed during shooting, and JPEG compression reduces microcontrast.
However it would be a total disaster for the (lower res) medium format cameras if they do not show an significant edge, costing many times more and being less versatile. Sony's sensors have shown to perform very well, so I'm sure medium format at similar resolution won't have much of an edge concerning the sensor. The value of having larger area is often greatly exaggerated - at base ISO photon shot noise is not a problem for 24x36, the difference up to say 44x33 (typical size of entry level medium format) is negligible.
The advantage of medium format will instead be on the lens side I think, where you can have larger pixels for the same resolution and thus stress the resolving power of the lenses less.
At 36 megapixels it will be very evident which lenses that are sharp and which are not. Which are sharp and which are not will depend very much on which optical designs that are possible for a specific focal length, plus how well executed they are of course. Some focal lengths will perform very well, while others will not. 50, 85 mm will be good, 70-200 zoom will be good. Ultra wide lenses will be poor. I'm more familiar with Canon lens performance though, Nikon are better than Canon on some focal lengths and vice versa.