@HarryT: Basic hardware inversion has been supported (... without actually crashing in some obscure corner-cases
) for quite a while (the latest NightMode hack versions actually rely on it, and I'd imagine Nickel does, too, as the switch in the hack was implemented by a Nickel dev).
What Amazon did is tailor a dedicated waveform update mode to that, as well as add some bit of frontlight trickery to make it easier on the eyes.
If you've ever used nightmode, you'll have noticed that ghosting is much, much, much worse, which is what they tried to combat (the waveform helps with ghosting, while the FL tricks help with black flashes, since they now become retina-searing white flashes).
I don't have the aforementioned devices to check, but I'd imagine they also made the full software stack aware of the current status in order to handle images properly, and not inverted
.
TL;DR: They went the extra mile to make it an actual working, useful, usable, user-friendly feature, and not a "hack"
.