The problem with Discourse oneboxes displayed in WordPress comments is that the Discourse onebox styles aren’t set WordPress. All of the Discourse onebox markup is present in the WordPress comment, so it’s possible to add the Discourse onebox styles to your WordPress site.
You can hide the body of the onebox with the following CSS on your WordPress site. This will just leave the onebox header - a link to the original article.
.comment-content .onebox .onebox-body {
display: none;
}