Most cryptocurrencies are in fact decentralized by definition, because there isn't an actual central authority that regulates it. The degree of decentralization is the point of discussion here. That has to do with how any entity is feasibly able to control Bitcoin, and the answer for that is that it is not easy. You can restrict access to it, through GFW or something similar but it isn't effective till you block it off entirely.

Even if you establish a hermit empire like NK, you don't compromise the decentralization of Bitcoin. You merely lose access to that part of consumers but it doesn't compromise the decentralization of it otherwise.