They finish with "how do you fix hardware", and my understanding would be "with firmware". I suspect that even though both devices are running the same version of iOS, aren't the firmware drivers for the chips still different, and can't they be updated?
And if it's the actual hardware, rather than the software/firmware, I guess they'll offer free replacements in the Apple Stores for anyone who has the problem? Sure, it's a hassle for the customer and expensive for Apple, but hardly the impossible problem to solve the author seems to want to make it out to be.
I think this is less of a deal than these journalists are making it.
Why is the 5S being tested in a case?
Because people like to buy beautiful things and hide them in clunky cases.