OPML allows your to export/import the list of RSS/Atom feeds from your RSS reader. So this is a list of 500-600 blogs that people submitted at that HN discussion, that you can import in your RSS reader.
OPML (when used in this context) is basically a list of URLs with a touch of metadata. It is the most common format for importing and exporting lists of feeds to move between feed readers.
So you generally don't subscribe to an OPML file, it just provides a list of feeds to subscribe to.