If he's using Python, he's likely using "feedfinder.py", by Aaron Swartz. It doesn't find your feed at all.
It looks like your feed isn't being served with an appropriate mime-type -- or at least not one that it understands. Seems to be failing in the 'test' method.
Never heard of anyone using this content type. It's typically the application name with XML as a modifier, "application/xhtml+xml" or "application/atom+xml". "text/xml" is the generic catch-all but is not valid for feeds.
It looks like your feed isn't being served with an appropriate mime-type -- or at least not one that it understands. Seems to be failing in the 'test' method.