There seems to be a general problem with “Ruby RSS” and “Media RSS extensions” as described here
I assume “discourse-rss-polling” uses the same Ruby implementation. I don’t know how “discourse-rss-polling” is related to the “wellfed-plugin”. Does this limitation exist for “discourse-rss-polling” as well?
The two plugins are very similar. The discourse-rss-polling plugin uses the same code to parse the RSS feed as the wellfed plugin. I tried adding the RSS feed that you linked to to my site’s RSS settings. It is not pulling in the feed on my site. I’m not sure if this is something that can be fixed.