You missed my point.
If I buy something on 28° MC, say in Euros. I get the MC rate which includes a small fee (< 0.5%). If I’d used my AMEX or NAB Visa CC, I’d pay their respective rates (pretty similar to MC), but those cards add 3% international tx fee.
Now, back that purchase. If the merchant selected AUD, I’d be charged > 6% additional.
What I don’t know (but assume you do) still pay 3% and 6% DCC on the AMEX and other CC/DCs that aren’t international fee free.
But in the end, no one other than the merchant and POS provider is financially better off paying DCC.