Creating new metadata example in Javascript Tool Guide

I'm working my way through the example but when I run the following code:

// load the library
if (ExternalObject.AdobeXMPScript == undefined) {
    ExternalObject.AdobeXMPScript = new ExternalObject('lib:AdobeXMPScript');
}

xmp = new XMPMeta();
xmp.setProperty(XMPConst.NS_XMP, "CreatorTool", "My Script");
xmpStr = xmp.serialize(); // serialize the XMP packet to XML

The console in EST returns

Execution finished. Result: <?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?>
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="Adobe XMP Core 4.2.2-c063 53.352624, 2008/07/30-18:05:41        ">
   <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
      <rdf:Description rdf:about=""
            xmlns:xmp="http://ns.adobe.com/xap/1.0/">
         <xmp:CreatorTool>My Script</xmp:CreatorTool>
      </rdf:Description>
   </rdf:RDF>
</x:xmpmeta>
                                                                                                  
                          
<?xpacket end="w"?>

But when I check the file info of the file open the <xmp:CreatorTool>My Script</xmp:CreatorTool> has not been added.

Am I doing something wrong?