I suspect that the poster you're replying to makes a confusion with public posts, which ActivityPub, unlike email, has. When the public namespace is encountered

I suspect that the poster you're replying to makes a confusion with public posts, which ActivityPub, unlike email, has. When the public namespace is encountered as a recipient of an Activity, Mastodon indeed iterates over all the instances it is connected to and pushes it to all. But for follower only posting, you are right, Activities go only to the recipients, theoretically to each individual inbox, practically to the instance's shared inbox.