Being technically decentralised doesn't make it practically so. The Web is technically a hypertext, running on a partially interconnected mesh network, yet nowadays the bulk of traffic flows between a handful of giant hubs, to the point where "marginal" social networks stay that way simply due to a lack of critical mass, and not having an account on some giant's servers is a communication problem for many. We have, and consider normal, major communication systems that only talk to themselves. XMPP is decentralised by design, yet when it was popular, Google was the main player and its changes were adopted even if they were unsuitable for most, simply because they were needed to interoperate with it. When Google abandoned it, the users vanished and XMPP essentially died, having become irrelevant.

To put it another way, yes, Nostr is decentralised by design, but this peculiar design makes it practically centralised, and it is, or rather will be in the future if it succeeds, a problem. Just see Primal as an example.