Okay - still not sure what the big deal of it is -It isn't really a big deal. But if you're a member of a lot of low-activity groups, or like to read a lot of blogs, it can be handy to set up all their feeds in a reader or aggregator, and thereby get a single digest that contains any recent messages from all those different sources.
RSS wouldn't really be appropriate, in my opinion, for keeping up with a very busy email list. For that a conventional digest from that list is probably the better choice.
but it sounds like the answer for the OP is to tell the member to justI'm not sure, but Mark may have addressed the actual question.
One of the available Integrations in a group is to post messages from an RSS feed (or maybe more than one). For example, if I had a blog where I post major articles, I could integrate its RSS feed to my group. This would cause a "blurb" (digest) of the article to be posted in the group whenever I make a new blog post. That might be appropriate if the blog and the group are on the same topic, and it might be a better way of having a user community comment on your blog than using the blog service's comment mechanism.
So if the OP's group has such an integration, then there may have been a member hoping to not receive the RSS "blurbs". Muting the #feed hashtag would be a solution for that.