“For optimal flavor, open the package 30 minutes before eating” - only temperature or also chemistry?
Some food is indeed packaged in a protective atmosphere. This is done in order to prevent the outer layers of the food from changing due to oxidation.

I am not aware of any food that would be vacuum-sealed or packaged in a protective atmosphere which would benefit from the chemical processes after the package is opened.

The reason why manufacturers suggest opening the package is most likely just to facilitate heat exchange with the surrounding air in order to bring the temperature up to improve the flavor and aroma of the food.