I may be totally off track here, but I am thinking that the <xmp> tag is only used for the parser to identify which theme the author wants to use? I am assuming that the end HTML just strips out the xmp tag and inserts the relevant bootstrap CSS/JS links in there to match the theme, so the actual xmp tag is never passed across to the browser?
Presumably it was chosen because it also allows the author to include HTML tags inside Markdown (somewhat common in Markdown usage) with the browser (mostly) ignoring them. It's part of why many other options using <script type='text/markdown'>, though at least in SCRIPT's case it hasn't been deprecated since HTML 3.2.
