If it's a 403 being sent, but otherwise the feed is visible, then I suspect they're running something which is designed to block certain requests, such as requests from non-standard user agents (that doesn't look like a browser, for example) or other criteria.
Indeed, after further investigation, the underlying response being sent is from the Sucuri Firewall. Not a lot that can be done unless you know the feed owners and they're willing to whitelist you.