What you said is about right. Just would like to add a few things.
Few seasons wood for 10 nowadays. Traditionally, seasoning means having the lumber stacked outdoor for X number of years. Today, to many makers, it means seasoning in electrical kilns. Other alternatives include using laminated wood make soundboard to avoid cracking and warpage翘曲, using particle boards for case, using plastic for action parts and many other parts, etc.
It’s not only the wood for soundboard need to be seasoned. Wood for many other parts such as almost all the parts of the action, the keys, the pin-block, the bridges and the case, if it is made of solid wood, need to be seasoned.
High-end European pianos are more prone to humidity fluctuations, though they are often more expensive than their American counterparts. Larry Fine mentions that in his book as well. I think it is because the climates oversea are not as harsh as ours in North America.
