This is flat out incorrect. It's actually quite difficult to get a cash FX rate that's even close to as good as the standard credit card FX rate, which is also set by Visa/MC and not the banks.

Where the banks do get their pound of flesh, though, is the foreign transaction surcharge, which is often around 3%. No-fee cards exist but you need to look for them. And a whole new set of charges applies to doing a foreign cash advance on a credit card.