In the good old days, TAs had sometimes access to better fares but with more restrictive conditions.
I guess that some Amex agents are reading you the text of these "old" restrictions for your fares..
As long as QR has a policy to refund any ticket, Amex will get the refund from QR.

What would happen if QR decides in a few months not to refund voluntary cancellation is a grey area.
If you booked on the website, you can easily argue that QR publicized a refund guarantee for new booking at the time of booking.
But there is a risk, albeit small, that Amex/QR decide to apply the standard fare rules linked to your ticket when issued by Amex..