This guideline is trying to help avoid some specific problems with using appendChild on unclosed ancestor elements in older versions of IE:
http://support.microsoft.com/kb/927917 (archive)
http://blogs.msdn.com/b/ie/archive/2008/04/23/what-happened-to-operation-aborted.aspx (archive)
You can avoid these problems by always placing your GTM snippet directly inside the body tag, not wrapped in any other elements (e.g. <div> or <td>).