Yes, this is an exoteric feature with an even more esoteric description:)
When I use Metacello to bootstrap GemStone, I have TrueRepositories turned off, because I am loading from a disk-based repository (using repositoryOverrides in the loader) so that we don't _have_ to hit the http repository for every GemStone product build (and for folks using GLASS behind corporate firewalls that don't have direct access to HTPP repositories).
As a result the repositoryGroups are empty for all of the packages. This is a real pain when you start doing development.
'Update Package Repositories' scans every package and adds the project repository to the repositoryGroup for each package, so that one doesn't have to manually do so.
I've updated the blog with this information (hopefully this explanation is clearer:)
Dale