Actually it makes sense for Nacra to standardize the mainsail size. The F20 class was dead, and so this standardization would creat a stronger worldwide class.

Also, hasn't Nacra petitioned to ISAF to adjust the F20 class to meet this spec?

It would be very easy for other builders to comply with the spec as well... just a new main, which people get every year or two anyway. If the F20 class is dead, its because of the cost of the boats compared to F18, and that you require 2 strong people, vs F18 which doesn't require as much physical strength.