My (and local inspectors) interpretation of this part of the code is this:
Feeding a separate structure from the main service can be done both ways.
One is the way described. Sepearate ground and neutral conductors and keep the grounds and neutrals separate in the subpanel.
The other is to run only the grounded conductor and no ecg. In a sense the new structure becomes another main panel. Ground and neutrals can be together. We run 3 wire URD for this purpose.
I don't have my books at home with me but I know it is in there in not too complicated terms. This exact scenario was debated in a code meeting with all the brass out of Albany, NY not too long ago.
Bottom line is if you run 3 wire it's treated as another main panel. If you run 4 wire it's a sub-panel even though it's in a separate structure.
Either way you need a ground rod (or two) at the separate structure.
For a purpose such as yours we often use a feed-through panel. This has a main breaker at the top with usually 6 or 8 spaces and main lugs a the bottom. This means the full load can be drawn off the main without the need for a sub-breaker.
