You can’t see any content by short codes either. Discourse should do PHP of WordPress in such situation, but it can’t so the plugin is showing only more or less preview/just text/images/”passive” content.
I don’t know anything but I can’t see how Discourse could parse PHP through a plugin.
As I see this WP Discourse connects a topic to WP-post as commenting system. WP Discourse is not an importer or mirror from WP-post to a topic as you are trying to use it now.