That may be the case although I’d expect the request that Discourse makes to the site to return HTML. Something else to look into is that Discourse caches the content that it’s pulled from the external site for 10 minutes. That means that if you are adjusting the allowed embed selectors site setting, you’ll have to wait up to 10 minutes to see the results of the new setting value.
If your Discourse site isn’t yet in production and you have access to the Discourse site’s Rails console, you can clear the cache from the console with Rails.cache.clear
Some additional details about the setting are here: Configure the Allowed Embed Selectors Setting.
Edit: I think that embedding is working as expected. It’s just tricky to configure for some sites. I’m going to move this topic to support.
I created a feature topic a few months ago suggesting that Discourse should find a better way of parsing embedded posts: Topic embedding needs some love. I’ll put some time into that soon if no one gets to it before me.