Short answer
HubSpot's KB: auto-publishing creates a social post 15 minutes after a new blog post publishes, once per blog post. It's only available on shared social accounts (Accounts access set to Everyone). Once on, it can't be turned off if the account stops being shared.
1. In HubSpot
The relevant screen in my demo portal.
2. Worth knowing
Preview and customize each auto post before it goes out.