Sound advice. I worked as a ASE certified automotive technician for years at a Ford dealer. Ford actually sent out a TSB stating these cracks were not cause for replacement.Cracks in the rubber like the one in the image are superficial. It's like saying paint cracks mean the wall is unsound.
The serp belt's structure is in the cords of the belt, and such cracks do not affect these cords.
If you start to lose this rubber, it still won't harm the belt structure, but that's when i replace it.
This is an advertisement from a particular Ford dealer trying to drum up business, not from Ford corporate engineers that sent out the TSB years ago, that I refereed too. However, I would agree that the worn belt they show in the pic is due for replacement. The pic of the OP's belt looks nothing like this one and has many good miles left in it.