There is a lot of markup on that page. I’m not surprised that Discourse is having trouble finding the main post content. Try this to see if it grabs the main content:
article .mvp-main-box #mvp-post-main
The results are cached for 10 minutes, so you will have to wait up to 10 minutes to know if the changes are correct.
You could also try just using article for the setting. It is possible that will work.