Thanks for diagnosing that for me, you're right blocking third party cookies does cause it to fail.
Both tests are equally valid. I just gave one because trying to be exhaustive about testing it would be mind-numbing. The test I provded only does localstorage, but FPI also isolates DNS cache, H2, image cache, favicons, cookies, localstorage, indexdb, etc etc
You can do yours by visiting
https://anonymity.is/misc/ff/fpi-iframe.html
first; then visit the ritter.vg and rittervg.com links.