Near 100%. If you look at the binwalk output in his later posts you can clearly see certificates as one of the first things in the binary. I’ll be shocked if this guy ever actually gets his own firmware to run here.
I've never really understood why Amazon doesn't want people hacking with their devices. If I were them, making a Really Hacker Friendly Device would boost sales hugely, as people would start making 'cool things' with it. All of their devices are incredibly tied down, however, despite the promise of relatively cheap computing (I don't own any of them, out of privacy concerns).
If some form of secure boot is used, then replacing the flash won't work either. There is one-time writable storage inside the SoC itself is used for verification. You won't be able to get it back into a state where it accepts non-signed firmware without also replacing the SoC.