If you look at the results from the W3 validation you listed, it reads:
line 8, column 0: Undocumented use of atom:link The atom:link element being placed before the channel element is causing the SyndicationFeed class to fail on load. You can test this yourself by downloading the rss feed xml locally, removing/commenting the atom:link line and running your code again. Without that line, the xml loads and the feeds are found. This has happened before with the SyndicationFeed class.