they don't make it easy
First, in regards to your reference to the 7210 in your article of yesterday 9/20 (Oracle promises fresh approach to storage). There was a 7210, but it looks like it got morphed into the J4500 (then X4500, and now X4540), which I think was coded Thumper. You have to look at the System Handbook to see this.
As for the 7720, my take is that it's all pre-packaged (as you've mentioned, a la Exadata, Exalogic, exacetera), and high density to meet "bulk storage and backup requirements". Sure the 7420 scales out farther, but it only gets 384TB in a rack, not 720TB.
It's also very confusing how they package the disks in the shelves for the 7720. If you look down in the data sheet, it says 30 drives per shelf (as opposed to 24 per in the other models), to get the max capacity. If you're thinking 8 shelves (like the 7420), this explains why they say "starting capacities of 240TB", with 1TB drives. It doesn't explain how they get to 720TB, which they say takes 360 drives, which means they're not getting the extra capacity with larger (than 2TB) drives. To get 360 drives in 8 shelves, you have to have 45 drives per shelf. Seriously? I know how they get 30 instead of 24.. the same way they get 12 blades across instead of 10 in the 6048 chassis vs the 6000 chassis.. they customize the rack, and use the space at the sides, and add another column of 6 drives to existing shelves. I think they're actually using 10 shelves in the 7720, with 24x 1TB drives in "modular" shelves, initially. Then V2 will be 30x 2TB drives in "integrated" shelves. The confusion is that 8x30 = 10x24. Now the 7720 starts to look like an altogether different beast.