Interesting video, I have to admit I looked at the part which show Flashing the Ledger Nano S with custom firmware just because I use that HW. In this part of video we can see that is possible to flash Nano S with custom firmware, and in case they presented we see that instead HW you can turn on this device in miniature game console and play game snake.
Correct me if I'm wrong, but at 17:00 onwards I see them succeeding in installing custom firmware and running it via the Bootloader only? They don't actually run any custom firmware which has access to the secure element, which is where your seed and PIN are stored.