I think you're misinterpreting this statement. I'm using nostr for more than a year and I can publish to any free relay I want to. On Mastodon your account is tied to a specific instance while on nostr you have a private/public key pair that's independent of any relay. There are some more focused/curated relays that have additional filters/rules or only allow certain people to publish (whitelisted public keys, often paid or invite only). I know of no mechanism on nostr that would force anyone to stick to certain topics or issues.

I think the point of the quote is that Mastodon tries to be both a topic-centered community platform as well as a "everything goes" public social network like Twitter/X but the federation aspect is not true decentralization because you can easily lose your social graph/reach if some instance admin doesn't like you or your own instance gets #fediblocked.