Is this maybe an apprentice looking for an answer on a journeyman practice exam? No? O.K., I'll take it at face value.
You're right in that a single branch circuit run to a separate building requires only a cutoff, not another grounding electrode system. But, by installing a loadcenter in the building, you're not running a branch circuit. You're running a feeder (because it's in front of the final OCPD).
The loadcenter, because it's serving a separate structure, is considered a service. Therefore, it has to be grounded at the building, using a made electrode system. The main reason, I think, is to bleed off a lightning strike.
Running an EGC is good, and required if there's any metallic path between the buildings. But it doesn't get you off the hook for installing a grounding electrode (i.e., two driven ground rods) at the second bldg. And if the building steel will be accessible in the finished bldg, you have to bond it to ground at the service. I'd bond it regardless.
Your apprentice is right. Look it up in Arts 250 and 230.
Best o' luck--