W3C validator says 'feed does not validate' 'url must be a full URL'... whats wrong with it?

The problem seems to be that it’s a HTTPS URL instead of a HTTP URL.

The linked error documentation, foo attribute of bar must be a full URL, says:

If this is a link to a web page, you must include the "http://" at the beginning and immediately follow it with a valid domain name.

The RSS 2.0 spec says about <enclosure>:

The url must be an http url.

If you change https://archive.org/download/… to http://archive.org/download/…, it validates.