[WorldMaker]

As I said, anecdotally I've already done it. Amazon only just enabled the PC "Send to Kindle" to support ePub directly instead of the old silly work around of rename the .epub to .kfx (and no other change). They've been very bad at keeping their list of formats up to date in their own documentation. Some of that perhaps because they don't want it to be so obvious and it is intentional obfuscation (to keep people using their store rather than going elsewhere for books), some of that because a lot of their kindle documentation seems to be in a "isn't broke, don't fix it" frozen state for years at time. You'll also note that the text you found doesn't mention "Kindle Format 10 (KFX)" at all and also you might notice that TXT and PDF are mentioned on both sides of that text as both "natively" and "through conversion" which seems to imply the original text was from the era when they were converted and they were added to the "natively" side later without remembering to clean up the other side. (They both have native support today.)

[chocochunks]

TXT and PDF are on both sides because Amazon will convert them to the appropriate Amazon format if you use send to kindle. TXT has always been natively supported on Kindle. As fair as Amazon is concerned, KFX is an internal format only for their use so there's no need to list it. When Amazon added ePub conversion officially KFX had already existed for over half a decade.

Your anecdote also seems to be the only instance of it working natively. Keep in mind Calibre will autoconvert for you.

[razorbeamz]

I was curious and just tested this with directly putting an ePub on my Kindle and it did not work.

I also tried renaming the ePub to a .kfx and it still did not work.

[amanaplanacanal]

I have also been unable to make this work. I don't know what WorldMaker is doing differently.

[chocochunks]

Having Calibre auto-convert it and not realize I guess.