Looks to me like the root node edge case has yet to be implemented though there are clearly comments regarding it. Looking at the code the comment seems incorrect in that the edge case is not being handled, rtaher than it has been handled.
// This fix ensures we have XML data
// Related to TreeStore calling getRoot with the root node, which isn't XML
// Probably should be resolved in TreeStore at some point
return Ext.DomQuery.selectNode(root, data); // ?fix I don't think so...