Ok, I was able to import a book I'd bought back in 2021.
Scribe 2022, firmware 5.18.5, jailbroken
macOS 15.7.1
Calibre 8.12
KFX Input 2.26.2
DeDRM v10.0.3 and v10.0.9
Book:
The Baroque Cycle (Purchased in 2021)
I used Content Library to have Amazon send the purchase to the Scribe. Sync'd and downloaded it to the Scribe then opened it to be sure it was working. Plugged Scribe into laptop and Calibre detected it. Initially tried to add the book to the library via the Device tab, but it failed as KFX-ZIP. Turns out I hadn't used DeDRM for Kindle stuff in so long that it was still at v10.0.3. I deleted the book from the library, updated the plugin to v10.0.9, and tried again. This time was successful and imported to KFX. I converted it to EPUB and opened it in the Calibre book reader, where it was fine.
So, it's not a foregone conclusion that Scribes on current firmware are downloading KFX with the new uncracked DRM.