That is most certainly true. In fact, I'd be shocked if Adele's performance was recorded nearly as well as Diana Krall's.
The difference in loudness is certainly due to the DRC.
The piano is really a "who knows" thing. I think it's DRC simply because I have never heard a piano that sounds that bed in any track with decent DR. Pure speculation and I wont pretend otherwise.
What I will do, mostly for shits and giggles, is edit the track myself to a DR of about 8 and see how much different it sounds.
![Big Grin :D :D](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
That would be a much better indicator of the damages done by DRC.