Hi Sara,
Yes, this is the intended behavior. Feedly can update older articles provided a feed has more than 20 followers. However, Feedly can only update articles currently returned by the RSS feed (typically 20-50), so it will not be able to update every old article that was published.
Unfortunately there's no easy way to update an entire feed right now: It would have to be a custom job and require some way for Feedly to fetch the entire feed.
In general though, this isn't a big issue: articles older than 31 days are auto-marked as read, and Feedly readers rarely look at articles older than that unless they saved them in boards.
I hope this helps.
Best regards,
--
David Chatenay
Platform Engineer
Feedly