The site at goodfoodcities.org doesn’t appear to be hosted by WP Engine. The meta data required for creating a onebox exists on that site, but when I try to create one on a live site, the request to the website is returning 404 Not Found. On my local dev site, oneboxes for goodfoodcities.org are created without a problem. This suggests that the problem is that the Discourse Forum Onebox user-agent is being blocked.
To check if a site is hosted on WP Engine, I view the source of the page and search for ‘wpengine’.