that's like saying that the internet isn't decentralized because lots of people interact using facebook. that address isn't decentralized, much like facebook isn't. that doesn't mean that bitcoin or the internet aren't decentralized.

"Decentralized" means little without more context. The issuance of bitcoins isn't a legal monopoly. In this sense Bitcoin is "decentralized". On the other hand the Bitcoin blockchain is a centralized ledger, which is distributed, but nonetheless centralized. So which one is it? Centralized or decentralized? It doesn't matter because at this stage "decentralized" is being used as a mere buzzword rather than to convey a precise meaning.