Earlier quoted context omitted.
With my mortgage, interest is monthly on the remaining principal and paying extra in a month is entirely on the principal - it reduces the total interest paid, so the bank gets less.
The idea that you pay the interest up front is a very common misunderstanding of how mortgages work and more broadly the concept of an amortization schedule.
The parent of that comment mentioning "a fee to overpay" is one I've never even heard of before. Definitely not the case here, free to pay down the principal as much as I want whenever I want as long as the current interest for the month is paid first.