Sono attivo da poco, ovviamente, ma conosco il protocollo e sto implementando un client.

Non credo sia errato dire che non è una piattaforma, ma nemmeno dire che lo è, in verità, dipendentemente da come si organizzano le idee.
Da una descrizione di Nostr che ho dato sul canale Telegram, in inglese, e che @fiatjaf ha approvato:

I think the simplest way to think about Nostr is this.

Suppose that you are a writer and I like reading your staff, so I follow you.
If you write on Twitter, Twitter can censor you, and I may miss on that post.
A solution could be: you write on Twitter, Facebook, YouTube (which now has textual posts too) and whatever other platfrom you want. You publish each of your posts on all of these platforms.
And I follow you on all these platforms.

This would work, and it shows that the web as a whole already is decentralized.
The issue with it is that it would take some significant effort from both of us, and I would see the same post from you multiple times.

Welcome Nostr.

Nostr relays act the same as traditional centralized platforms, except that they all offer (kind of) the same API.
With just one Nostr client, publishing the same post to multiple relays is trivial.
And if you follow the same user from multiple relays, you see each post only once. And you only need to use one interface.

So while each relay essentially behaves as a platform, actually this is mostly transparent to the user and from the user's prospective (which is the most important one) the whole Nostr echosystem is essentially just one decentralized platform. 

 Who wrote that? 

 I wrote that on Telegram. You commented with "this is very good".

I don't know if you changed your mind in the meanwhile. 

 It is so good. Very very good. Can I use it? 

 Of course you can, I'm using Nostr, t.

Do note I might use it myself as well on my blog when I talk about Nostr.