Cough, you are correct, but that same Wiki entry gives a table (which I used as reference) quoting both aircraft with the same wing area of 122.4 m^2.
Other references I have checked also give the A321 wing area as 122.4 m^2. Maybe it is a case of keeping the reference wing area the same even though there have been minor changes.
Whatever, I still believe that the basic reason for differences between A320 and A321 green dot speeds is an increase in zero lift drag, to which the increased wing area skin friction would contribute