Has nothing to do with decentralization. All the Feds need is a backup/scraper for nostr. The relays need warrants to help then track by IP if manhunt. But easily use Tor to skirt that.
Discussion
The other big difference is social media like Facebook track everywhere you go via mobile app, cookies, browser fingerprinting, 3rd party data sharing, and so on. So when asked for data kn you there is more there. So they get data of GPS and where you have been as example. Comparing that to nostr is like a .22 to a Howitzer.