Yes, true, and as the paper points out they are related. But the mining centralisation story is basically "water is wet" in 2020 for people interested in this. And as a many commenters here show, the mining centralisation problem is being thought about in technological ways ($BUZZWORD)...
The paper seems to focus more about the political/organisational problem, which to me seem more inherent and harder to solve.
Exactly. Bitcoin governance can still be called decentralized but there are still certain degrees of centralized control in decentralized systems (just look at the Internet). The political/organizational problem is hard to solve because humans have to coordinate to make decisions and often this is done from (centralized) points of authority (Lead Developers, Mining pools, Bitcoin wallets/exchanges). This is not a direct democracy but a represantative one because not everyone in the network is equal: 1) Core developers elect the Lead Developer to make decisions on protocol rule upgrades, 2) miners increase the likelihood of coin rewards by using mining pools and in the process elect mining pool operators to make voting decisions on their behalf, and 3) users use Bitcoin wallets to take on technical procedures associated with accessing bitcoins and in the process elect them to make lobbying decisions on their behalf (instead of running their own Bitcoin node). This creates a type of decentralised structure with centralised pieces. "The cost for collective action is [some form of] hierachy" (481).
The paper seems to focus more about the political/organisational problem, which to me seem more inherent and harder to solve.