As the standard clearly states:
For compatibility, the string " -- " (double-hyphen) MUST NOT occur within comments.
no decent parser should accept your 'XML' as well-formed. You may search for a faulty parser - this indicates that some version of BeautifulSoup (3.08) may accept -- in comments - but the real solution is either to clean the data before .loadXml or - better - to take a big stick to the author.