Ahhh, that's a helpful explanation, thanks!

So with Nostr - it's decentralised to the point that I'm (me the user) the individual point that's sending et, rather than Mastodon which is decentralised less such that there's multiple servers with many users.

That is correct. Plus, these are just plain text messages in the end of the day.

So you can always download and read them easily. I'm an old person from the forum days and was really annoying that whenever a forum would go down, all the useful posts and info shared over the years would disappear too. With this kind of mechanism, it is easier to rescue that data.

NOSTR isn't just about twitter-clones, it can basically be used to replicate blogs, forum and chat apps that exchange messges.