Making guitar soundboards is a lengthy task. First, trees with very straight grain are chosen this means very little twist to the tree. I know of a guy not far from where i live who goes out to the block where the logging is happening and he will use an ax to split the wood and see how straight it is. Then those trees are brought back and quarter sawn, making wedge shaped strips of the tree. Each piece is carefully inspected and pieces with defects such as knots are discarded.
The pieces then get bookmatched and hung to dry, and while wood generally takes 1 year per inch of thickness these actually are hung by clothespins for 2 years, if I remember quickly. These are then packaged and shipped after another close inspection.
I also assume that wherever the soundboards are being used to build guitars, that they are given time to balance the humidity of the new room.
Another factor is that the wood is very thin, and therefore there will be much less wood movement than there would be in a 2x4. Wood veneer is known for being stable for example. With such thin material you don't end up with one side being dryer than the other, so you don't have excess shrinkage one side over the other.
What verse is this if you remember off hand?