Good catch - so in effect IE7 does not support Netscape RSS 0.91 (which includes a DTD). Whilst Netscape RSS is a bit outdated, it is still in widespread use and supported by virtually every feed reader out there.
Realistically, the only way to get round the issue is if the feed was switched to Userland's RSS 0.91 or RSS 2.0.
See this MSDN reference [msdn.microsoft.com] for more information on why IE rejects DTDs.
IE7 and the Windows RSS Platform support the following feed formats:
RSS 2.0 (http://www.rssboard.org/rss-specification) RSS 1.0 (http://web.resource.org/rss/1.0/spec) Atom 1.0 (http://www.ietf.org/rfc/rfc4287.txt)
Additionally, IE7 and the Windows RSS Platform support the following formats that are considered deprecated by their authors. We encourage publishers of feeds in these formats to upgrade to current feed formats.
RSS 0.91 (http://backend.userland.com/rss091) RSS 0.92 (http://backend.userland.com/rss092) Atom 0.3 (http://www.mnot.net/drafts/draft-nottingham-atom-format-02.html)
I haven't seen many new sites deploying anything other than RSS2.0 or Atom1.0 recently.