The part about setting a printer on fire (a la the HCF instruction) is speculation - in HP's case at least there's the thermal breaker that prevents the paper from igniting, and I don't believe any other printer manufacturer would exclude this simple safety device (a paper burn from a stuck printer drum is easy to imagine), but the rest of the demonstration doesn't give much to worry about - a hacked printer can't do anywhere near as much damage as a hacked router, for example.