Have you tried this? Writing a specific onebox for your case might allow you to supply a onebox image to the cooked post? Then thumbnails would then work automatically.
I note that Youtube oneboxes I believe are static in-site content, until you click the play button and then it shows an iframe. The content presented prior to the click includes an image which is picked up then thumbnailed. Obviously Discourse can’t read from an iframe, so this technique is a good approach.
I note your example includes an og:image in the header tags which is perfect.
My suggestion is move away from javascript here and get cooking in Rails.
The only downside to this will be your image will be static until you rebuild the Post, assuming the target image is also updated. Thus if you are hoping to show a dynamically changing thumbnail you might have to get even more creative.