XFDF is a format you can only use when using AcroForms, it's not compatible
with XFA forms. Both are XML based, but that's just like saying both
English and Spanish are based on the alphabet, just because you know one
does not mean that you can speak (or in the case of XML - process)
the other one.
You are trying to use the JSObject, which does require that
you understand the JavaScript that the JSObject is based on. Do you know
how to execute Javascript in Acrobat's JS console? If so, then load your
document and execute the following command in the console:
this.xfa.data.saveXML();
This will output the XML structure for your fields (you will need to
reformat the XML to make it easier to read):
In this case, you can also get the contents of TextField1 using this line
of JS code:
this.xfa.form.form1.TextField1.rawValue;
And, you can set it via JS using this:
this.xfa.form.form1.TextField1.rawValue = "ABC";
All you have to do now is to use this knowledge when you
access the document data via the JSObject.