I wonder if they do bin them though, just not for iPhones or iPads. Perhaps today's A13/A14s that have small defects are tomorrow's Apple TV or HomePod CPUs.
A quick look in ifixit's HomePod teardown (2018) reveals an Apple A8 processor, last seen in the iPhone 6 (released 2014): https://www.ifixit.com/Teardown/HomePod+Teardown/103133
I wonder how long they manufacture their processor lines for.