Bitcoin decision making is channeled down a funnel: Core Developers make suggestions and the Lead Developer (and those given commit access) sign off on those decisions. Those decisions are then voted for by miners who are (relatively) centralised in that roughly 5 mining pool companies control the vast majority of hashing power used to vote on those decisions. Meanwhile large wallet/exchange companies who control vast amounts of on-chain transactions can lobby miners to pick certain decisions by upgrading their nodes to reflect new rules (miners will want to follow large companies because they create liquidity for coins with the new rules and so they can sell their coins more easily and, theoretically, for a higher value). So while this is still a decentralised systems because multiple parties have a say, there are still lots of points of centralised control in the governance system. In other words, not everyone is equal. "Individual developers submit to those with commit access, individual miners submit to mining pool operators, and everyday users submit to Bitcoin companies" (478). The Lead Developer acts as a centralised decision maker, mining pools act like centralised voters, and Bitcoin companies act like centralised lobbyers. So there is a certain structure to Bitcoin governance.

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."