I think he misspoke and meant "XHTML" rather than XML. While you are correct that XML allows upper-, lower-, and mixed-case tag names, the XHTML spec does specifically require lowercase:

4.2. Element and attribute names must be in lower case

XHTML documents must use lower case for all HTML element and attribute names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are different tags.

http://www.w3.org/TR/xhtml1/#h-4.2