Correct. Notes can potentially be rebroadcast to other, free relays and spammers may see them and, say, comment under them using the free relay. But as you’re only subscribed to your paid relay, you won’t see the spam.
Discussion
Just sucks that spam-filtered free speech doesn’t seem possible without a cost to the user. But if that’s just a law of nature, the nostr way seems like the best option. Certainly better than a centralized platform at least