I didn't say it was private, I said it wasn't public, there's a difference. If you asked me what number I was thinking of I'd tell you, but that's not the same thing as the number I'm thinking of being public information. ActivityPub is, at its core, about consent. We have consented to having our data be sent to any person able to serve 200 responses on an inbox endpoint by using instances with open federation. We could, if that makes us uncomfortable, moved to a closed federation system where we only accept request from an allowlisted set of instances, with software that follows the spec's public addressing system.