You're reading that right.
At ISO 100 the D5100 (and D7000) are very good low ISO performers.
These topics are often discussed in forums in terms of technical ability of the engineering teams, and that may be playing some role, but my take is that cameras most often perform exactly the way the company selling them wants them to perform. In other words... what are the important parameters required to achieve the business goals that have been set for a given model?
The product planning is a tough job for camera manufacturers...
- how to prioritize, among others, the 3 following variables: resolution, low ISO DR, high ISO image quality,
- how to define the target that needs to be achieved for these metrics relative to both the want of the photographers and their actual needs
- how to please experts and the general public
- video vs still
- ...
It seems that, 6 years ago when specing the 5DII, the product planners at Canon were focusing on resolution, high ISO noise and video. It can be argued that the 5DII reached a very good balance at the time being and that its spec were a good match for both the wants and the actual needs of a large set of photographers... except landscape shooters for whom DR was lacking.
It seems that, 3 years ago when specing the 5DIII, the product planners at Canon decided that their main problem was Nikon's superior high ISO image quality and that their main differentiator was video. It seems that they didn't acknowledge the relevance of low ISO DR.
Now, is there real value in going farther than the D700/D3 in terms of low light image quality? I am sure sports shooters would answer yes... I am not sure wedding guys really need anything better although they may still want it. But I am sure pretty that landscape guys need more DR...
Cheers,
Bernard