I dont know if it would make a difference. She might just do it out of habit. Or it was the "only" way she knew how to process foreign card. I have similar experiences at Poland railway station where the lady simply shrugged and told me that was the "only" way she knew how to do it despite her co-worker handled the transaction correctly earlier in the day.
Mind you, the transaction amount is small, like 14 euro in total I think. Not worth to spend time and effort to get it straight when the line was very long and moved very slowly despite we were the only ship in town.
FYI it is coded 1x on the CSR.

On all other transactions during the trip, primarily restaurant meals and B&B payments, some supermarkets, when I told them to use Euro, most seem to fully understand. Not a single attempt of DCC in Barcelona that surprised me. All Trenitalia ticket windows charged in Euro. All attractions ticket offices in Sicily also charged in Euro.

We even successfully get Avis to remove the automatic DCC part from the final bill as the agent at Pick Up did not remove the default clause just verbally assured us it would be in Euro. We knew better than that.