Raspberry Pi limits the input voltage with a Zener diode across the input. In a Zener used this way, the current flow through the diode varies with the voltage difference between the input and the output - so with enough voltage input you can still make it fail, as you can exceed the max power dissipation of the diode and then it will burn - and after that, your circuit is not protected anymore.
I can confirm that you can indeed destroy an RPI0 by accidentally connecting a power supply to the USB port while another power supply is connected to the power supply port.
