Discourse oneboxes do work with WordPress sites. If a link fails to onebox, it is likely because either the page it links to is missing the necessary Open Graph data, or because the request from Discourse is being blocked by the website’s server.
You can check the Open Graph data for a URL here: http://iframely.com/debug.
The website you are trying to link to is hosted by WPEngine. They are blocking requests from the Discourse Forum Onebox user agent. To allow sites hosted by WPEngine to create oneboxes on Discourse, the owner of the site needs to contact WPEngine and ask them to allow the Discourse Forum Onebox User Agent. If you are the owner of the website, it would be great if you could contact WPEngine support and ask about this. We would like them to allow requests made with the Discourse Forum Onebox User Agent by default.