As you noticed, groups are exported, expanded or not, as they look in the furniture list.
It's probably something which should be improved, moreover since there's no way to distinguish a group from other items in the CSV file.
As I don't know that much how people use CSV exports, it could be also nice to share how you use or want to use a CSV export, to help me improve this feature.

I am designing a motorhome and need to export the furniture list for an import to excel where I calculate for the used materials, the quantity (m2, pieces, packages), the weight and the cost. That means nothing that should be done within Sweet Home 3D.

Helpful would be a function in the context menu of the furniture list for "expand all" and "collaps all". All means all groups and subgroups !
Maybe this could be done by an plugin.