This is correct. I have a 7900 and can verify I get the same problems (though I don't really do anything with the QTR curve creation tools since I print with Piezography curves).
QTRGui works just fine for printing with 10-channel printers like the 7900, but the Curve Creation part of QTRGui is NOT functional for 10-channel printers. As Brian says, it was written many years ago and the Curve Creation part is not coded to handle anything over 8-channel printers.