A humidity control system not only keeps the piano more stable between tunings, preventing excessive wear on the tuning pins, but it really does extend the piano's life. The tuning pins on a piano are not like a guitar, where gears hold the tension: they're like a violin, simply a friction fit! There's a thick piece of butcherblock under the plate that the pins are driven into. It's simply friction that holds the 150 pounds of tension per string.
If that butcherblock (called the "pinblock") delaminates or splits in such a way that the tuning pins can no longer hold the tension, the piano must be completely rebuilt, removing the strings and the plate to replace the pinblock. (There are short-term fixes that can be applied first.)
So controlling humidity really does extend the life of a piano. Most humidity systems attached to furnaces only add moisture when the furnace is running, so if the temperature is OK but the humidity is not, the furnace doesn't run, and the humidity drops.
The site you gave belongs to a technician that installs Piano Life Savers. The company's site is here:
https://www.dampp-chaser.com--Cy--