@rtiangha: Don't rely on old documentation.
Freetype's behavior re: hinting has changed recently (-ish), and how it was built can affect the results anyway. (And even on desktop Linux, where you mostly control the full rendering chain, getting stuff to do what you want can be a bit maddening).
Because the autohinter can be configured in a couple different ways, and the defaults and how some settings interact with that have changed recently (again, -ish).
Amazon, at least, is using a heavily patched FreeType build, so, who knows what's happening there. The font hack used to be able to override it with a custom build with more control over the hinting settings being used, but that's no longer possible.
As for Kobo, they may be using Monotype's iType in some instances, and/or Adobe's own rendering engine, so, also murky.
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On recent, default Freetype builds, the priority is indeed BCI > autohint > no hinting. And you do explicitly need to request no hinting.
ttfautohint will just bake FT's own autohinting into the font as truetype instructions, which is dubiously useful if the final renderer is also FT (it's basically only useful for old fonts with bytecode that FT doesn't deal with properly, when targeting low dpi rendering).
On high-dpi devices such as ours, you'll generally vastly prefer unhinted rendering to autohinted rendering.
Random related plug: KOReader will allow you to try all three rendering types (with the autohinter set to the new defaults, slight hinting, with a grayscale target screen).
For instance, I am happily using Bookerly rendered unhinted on my Forma, and it doesn't look like crap
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TL;DR: Yeah, Amazon fucked up on that one. They should have enforced unhinted rendering, too.
(Fun fact: this is what's done on Android. And iOS. [Roughly]).