I think that statement is too broad, too general, and it needs to be clarified.

Using a home country DEBIT card to make withdrawals from your bank account at ATMs while abroad is probably the least expensive way to obtain cash when away from home, especially if the funds you are withdrawing are not in the same currency as your home country currency. You will often be charged a nominal amount - typically about USD $2 or $3 - by the local ATM operator for the withdrawal transaction (regardless of amount withdrawn), but because the currency conversion is done by your home bank you will get the most advantageous currency conversion rate available. So the trick is to not make frequent small withdrawals, instead, make occasional large withdrawals.

Using a CREDIT card to obtain cash is never a good idea, not even in your home town, because your bank and/or credit card issuer will begin to charge you daily interest immediately on the withdrawal - it is treated as a cash advance on your credit line, not as a withdrawal from your bank account.

Using a credit card to make purchases in a currency other than your home country currency will probably result in a currency conversion surcharge of typically 2% on the value of the purchase. This might not be a concern on a small purchase such as a tank of gas, but it is something to consider when making larger purchases in the hundreds of dollars or hundreds of Euros, Pounds, etc.

Michael