Flipboard subscribes to RSS feeds on our servers -- the Flipboard application requests the latest feed items from our own service. Our service stays up to date with RSS feeds through either just polling, or Pubsubhubbub push (plus the occasional poll). There is (currently) no way to force a poll through the application. However with Pubsubhubbub of course our feeds will be up to date very rapidly after an item is published.
I show that our system is currently subscribed to your feed via the appspot hub, but that we have received no pushes. In addition, I have purged and re-polled your feed, and we currently have seven articles. As Julien has pointed out in other messages, there is not really a widely adopted mechanism to indicate that an item should be deleted from a cached copy of a feed.