I encountered the same problem and has no luck at all.
The machine is a Kindle Touch bought 1 month ago, with no firmware upgrade. And it was brick while moving some books to collection. Keep hitting the tree and reboot itself, no progress bar, nothing.
I can successfully enter the fastboot mode and flash img into the kindle, but failed to enter the diags and keep looping the Tree. it's attempted to boot but soon reboot itself, but never going into diags menu.
This is what I've done in fastboot
1. fastboot flash diags mmcblk0p2_ssh-kt500.img (I got it from a mirror claiming that it is 5.0.0, too bad gitbrew is dead)
2. fastboot flash diags_kernel kt_5.0.0-diags_kernel.img (this is from the repository)
3. fastboot flash kernel kernel-5.0.0.img (this is also from the repository)
4. fastboot setvar bootmode diags
5. fastboot reboot
I believe the potential problem might be the version of diags img, then I tried all other available diags img on the internet, regardless of their versions, but none of them is able to bring me into the diags menu.
Any alternative solution for me? Please.