The implication was that concentrations of wealth are not a law of nature, but rather an emergent property of how capitalism is set up to be. This idea is not tautological, just a casual relationship.

Regarding the 0-sum aspect, I don't know how else you can look at it. Ownership of capital has to follow a certain distribution, which can in turn be more or less egalitarian, depending on how we decide to set the system up. it's however not possible to have both concentrated ownership and co-op style ownership at the same time (for the same company).

To be more specific, it's all down to how the society understands and enforces property rights. If land and real estate can only be held in usufruct, for example (i.e. if society only protects violation of those rights), then capital cannot be concentrated indefinitely.