In that example, the is_staff bit is part of the function of this particular query. It’s specifically added here SELECT user_id, true as is_staff rather than coming from the database itself. It sets anyone from the group you designate as ‘staff’ so they can be split into the two result sets (staff posts versus non-staff posts). 
So if you had a group for ‘employees’, which wouldn’t technically be site staff as the database sets them, you could still add them and they would go in the ‘staff’ bucket and not the ‘non-staff’ bucket.