The wrong fluid is the wrong fluid. Period. If your mechanic filled your transmission with brake fluid and it broke, would he question whether the fluid was at fault? If he filled it with washer fluid would anyone question whether the fluid contributed to transmission failure?
Cars are engineered with some very specific and important fluid specifications and to not follow them is going to result in mechanical failure. Now your mechanic made an honest mistake, as everyone does, but my opinion is he owes you a replacement transmission of about the same age and mileage as yours.
Nissan CVTs have some reputation for early failure but in my opinion the Hondas have proven to be quite durable.