It's one of those "no censorship at any cost" type things that are then surprised when they have nazis. Also a lot of crypto stuff. Just look at this trending page: https://nostr.band/?trending=posts
While ActivityPub is a way for social media instances/server to ask for a user feed or do updates (likes, comments, etc.), Nostr is a media format and more client-side protocol.
ActivityPub. Servers talk to each other, users are talking with servers in different language, like Mastodon talking Mastodon API. Users log in to their servers and have an ID there.
Nostr. Users have ID for themselfs, servers only act as a proxies to deliver a message, feeds are stored on devices, account do not live on any single place. Both users need to have at least one server common for each other to see their feed.
Honestly I think while ActivityPub is a nice glue, Nostr is already obsolete.
In my opinion problems that Nostr is trying to solve should not be dealed with making an Peer2Peer social media app, but rather with universal P2P network.
Decentralized networks like Yggdrasil or I2P do anything Nostr can do and are not limited to only posts, but can deliver any data just like the Internet. I host my own Minecraft server on Yggdrasil for example.