That line is a psychological line. http is a decentralized protocol, tho some will argue that it is not. But if it is the parent protocol of nostr, so anything nostr can do, http can do, and more. So if you think nostr is decentralized then http must be decentralized.

It might be useful to use the term "sufficiently decentralized". The term has become a slogan and a cliche in modern s/w engineering, in any case.

You could look at it another way. If the main bluesky node went down it could be critical. If damus went down it would be a pretty bad situation in nostr. If one particular website goes down it doesnt matter too much. If any bitcoin node or miner goes down, it will barely be noticed. If one pubky or bittorrent node goes down, the system is resilient.