> In a sense, data ownership on traditional platforms like Facebook and Twitter is governed by a terms of service, but on Nostr, it feels somewhat closer to publishing to the public domain. 

TOS are a contract. They only mean something because of the law.
Copyright law and privacy law still apply in the absence of a TOS document.
Of course relays and other parties can break the law, but the same would apply for mainstream platforms.

> Someone could, theoretically, be backing up your tweets, sure.

It's not theoretical, it's very doable.
This has been done before.

>  But large platforms do quite a bit to prevent that,

They can't prevent that. It's why there was an archive of Trump's tweets, for example.
Also tweets end up all the time on the Wayback Machine by the Internet Archive. I have accessed deleted tweets before this way.
In fact, backups of large portions of social media platforms have been made before by data hoarders.

> it's against their terms

People can break TOS.

> who owns your words after you say them aloud?

If they are copyright-worthy and recorded, the speaker owns them.

Mainstream platforms sometimes give users the delusion that they will be able to take things back, but they won't, or at least there is no guarantee that they will.