[danans]

From the article abstract, it appears that this paper is more about how the governance behind modifying the bitcoin code itself is hierarchical, and not the separate but dependent issue of mining power for the current protocol being concentrated in relatively few hands:

"The overall political framework for altering the Bitcoin code is described as senatorial governance: a (de)centralized model of bureaucratic parties who compete to change the monetary policy (codified rules) of the protocol. This model shows how Bitcoin is not an autonomous system but is assembled and maintained via human discretion."