Both moles and molarity can be used in this situation...many people/teachers prefer using moles, because it's easier to picture one H+ and one OH- neutralizing each other, than it is to picture 0.1 M of H+ and 0.1 M OH- neutralizing each other.
That's why you get the same answer either way (only applicable for strong acids and strong bases...weak acid or weak base would make things SLIGHTLY more complicated...but not too much)
Anyways, what you've done is correct in my mind. It seems highly likely to me, that there was a mistake in the question - not necessarily from you, bao....from the textbook/wherever you got it from.
Hope that helps