Hi Rowan,

I've done some testing regarding this, where I've tried to pull your rss feed into our EasyBlog site and it fetch the blog post correctly. http://take.ms/YVP6C
As you give the validator result above, actually the rss feed is still valid and WP should be able to fetch the rss feed.
Perhaps, can you try to fetch any Joomla's article rss feed and see if you can pull the rss feed or not into WP. If you still unable to fetch the rss feed article, perhaps you need to consult the issue to WP