It’s really not staking. The point is not ownership. Notes are information. Information can’t be owned, only hidden with encryption or tied to a public key by signing with a private key.
You are absolutely correct that storing your notes on someone else’s computer is just a promise. The great thing about Nostr is that if one of your relays goes down or decides they don’t like your notes you’ve still got 11 other relays still storing your notes. There are also plenty of one tap options available to start running relays that make it incredibly simple to self custody.