Why other card-payment news can confuse this
Seoul has added new machines that accept foreign cards for certain ticket and Climate Card purchases. That does not automatically change the ordinary T-money top-up rule at every convenience store or machine.
The safe arrival plan
Obtain a modest KRW amount, buy the physical T-money card and add a staged balance. Keep a card for rail tickets and mainstream purchases. If a new machine explicitly offers foreign-card loading for the product you are buying, follow the machine’s current instructions.
Do not over-load to avoid inconvenience
A larger one-time balance can create a refund problem at departure. Use the calculator’s transit estimate, round it to a practical amount and recheck after two or three days.