Indeed, Commons pages don’t produce nice embeds on Discourse. I don’t know what would need to happen on the Commons side to make this possible out of the box, but I guess it’s related to oEmbed or Open Graph support.
In the absence of this, it is perhaps possible to extend Discourse with OneBox custom code to support Commons.
Maybe this code to embed Commons videos on Phabricator is useful?
Also, I found this, slightly related, where Discourse contributors submitted a patch to improve Commons images embeds: