It's not just same content like
http://download.wsusoffline.net, it's done by filtering from
http://download.wsusoffline.net, and as soon the site will be updated, feed will do.
But comparing web page with rss feed is like comparing nntp protocol with imap. They just have different purposes!
Simply put, in Opera (and most other feed readers) you can not add download.wsusoffline.net, you must add feed address! Fortunately, recently Google Reader allowed to add just pages, but such auto-feed may not work as expected.