In the current version of Tribler, the actual downloading is anonymous but NOT the metadata fetching of the torrent. And, that exactly what the site you used is doing, checking only for the clients who requested the metadata for its magnet link. The site does not show peers which are actually downloading the torrent. Since there is nothing to be downloaded from the site's magnet link, I would say this test is not a sufficient example to show the anonymous downloading capability of Tribler.
Full anonymity (including metadata fetching) will be added in the future versions after Tribler 7.0 release.