I tested a number of tweets with the above example linked page after making a number of changes to the meta tags to change content and card type. Nothing appeared to work.
In re-reviewing the documentation, I see mention that the card contents are cached. I then tried tweeting another page and that worked. A new tweet attempt of the above page worked properly when I added a query string parameter to bust the cache. I suspect some bad meta content from an early test has been cached and that caused the card to not show.
