Nothing is truly "undumpable", but generally "undumpable" means "you can't get code execution with read access" which is not the case here. This is just a case

Nothing is truly "undumpable", but generally "undumpable" means "you can't get code execution with read access" which is not the case here. This is just a case of using the simplest possible output peripheral you can find before you can get anything else running. Canon camera firmwares have been dumped via blinking the LEDs. I've debugged low-level bringup by blinking codes out of LEDs too. This has nothing to do with side channels, it's just a case of not having documentation for the hardware so you find the first thing that gives any feedback and use it to dump the rest of the firmware so you can reverse engineer it and figure out how e.g. the screen works.