Yes, the PAGE field is recognized in MS Word. To elaborate more on the case, the custom field defined in the example you have found is not defined in the fields that MS Word recognizes and can be used only in RadRichTextBox. If you would like to show the current page number, you can use the PAGE field and NUMPAGES will show you the total number of pages in the document.
Hope this is helpful.
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.