The researchers found that while inequality did often increase with population growth and more hierarchical governments, this trend was far from universal. In some cases, human communities developed systems that curbed the concentration of wealth, using governance and cooperative institutions as “leveling mechanisms.”

What makes an effective leveling mechanism?

Edit: decided to look at the study, seems like they were looking at whether governance was “collective” vs “autocratic”:

For apical sites in this sample that are the central place (generally cities) in hierarchical polities of three or more levels, governance clearly matters for the degree of inequality. Only two of the 29 apex sites in hierarchical polities of three or more levels (Xochicalco and Tenochtitlan) that had collective forms of governance also had Gini coefficients larger than (above the) the regression line for autocratic sites at the apices of polities of the same ranks

(apparently higher Gini coefficient means more inequality)

Also they’re saying herd animals and metal play a role:

We must be careful about causality here. We are not proposing that herd animals, metal, or the control of trade routes directly caused the concentration of power and wealth, but rather that when governance institutions and practices were in place that did not check, or even fostered, the consolidation of power, those external resources facilitated the accumulation, monopolization, and personalization of wealth.