Yeah I can't say ActivityPub hasn't influenced Nostr, but it wasn't a conscious influence at any point if I remember correctly. BitTorrent trackers, SSB and nostr:npub1c878wu04lfqcl5avfy3p5x83ndpvedaxv0dg7pxthakq3jqdyzcs2n8avm's Diagon Alley were more of a direct influence.

But there are some similarities indeed, and if one wants to use Nostr like ActitivyPub they can easily associate themselves with a community relay, let the relay hold their keys on their behalf and go from there. Ditto is another approach that's close to this.