I think this is a very "high level" speech on domain hierarchy and the comparison is a bit confusing.
Basically when talking about a domain model hierarchy we have Objects, they are represented in a hierarchy tree in order of relevance as containers. Also, All Domain Objects are objects, but not all objects in a domain model are "Domain Objects". Others, such as OUs are just "containers" that belong to a domain hierarchy (thus can be child from a Domain Object parent) but not the opposite.
For instance, I remember a teacher explaining as the "restaurant" model. A Restaurant can contain goods, tables, and customers. Tables can contain customers, or chairs, but they can't contain a restaurant (example for Domain Objects). Furthermore, there are other objects related to the restaurant (domain) that are not related to the stablished hierarchy, but still are containers for other objects, for example register machine contains receipts or cash, or a booking log contains names, telephones and dates/times.
Hope this was not too metaphysical, and I helped to make it a bit clearer.
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