Kindle Previewer 3 and Kindle Create don’t even use KFX but KDF. Even if you retro-engineer KDF, there will be subtle changes making huge differences in KFX.
They already did that for Kindle iOS: AZK was a temporary format which lifetime was ± 5 minutes, born in Kindle Previewer, sideloaded via iTunes, converted to KCR in Kindle iOS. But at least there was a “bridge.”
For KDF–KFX, there isn’t. So basically, you can’t even test it on real devices, you have to trust their Preview — what could go wrong?
Also, it isn’t a concern because we let them do. It took an awful lot of activism (and public shaming) on the web but unfortunately, ebook producers are very few in comparison.