They offer a (fairly generous by industry standards) trade in program.
They take thousands and thousands of perfectly working iPhones, then turn them into raw scrap, just so they aren’t floating around on the secondary market.
Apple realises that most iPhone users aren’t going to switch to android, so reducing the supply of secondary market iPhones, really helps them sell more new devices.
They also scrap a whole bunch of them too[1].
While undoubtedly many of the shredded devices are beyond repair, many of the devices destroyed in the video have no obvious physical damage.
I don't think they release any numbers on the amount of traded in devices that are refurbished and destroyed.