If you factor in the resale value of a 2 year old iPhone and a 2 year old Pixel, I doubt this is true. And one of the biggest reasons for the high depreciation is the loss of software updates...
I think more than software updates, iPhone has an aspirational value in many parts of the world so there is good demand from people who can't buy a new one. Pixel in itself doesn't hold that value and there are manufacturers who will have an Android phone with Pixel's specs in half the price of Pixel within a year of its launch.
