As someone who hosts a public relay--I agree.
Do not send your personal shit to the superfriends public relay (or any other public relay, for that matter). I always encourage anyone to host their own relays for DMs and other private usage.
Nowadays you can spin up your own private relay with #haven or something else for $0, or very close to it. Use that for private notes, drafts, and other things you don't want exposed to the public.
With the outbox model there is no benefit to using public relays for private storage, only risks.