@ Harvie, Mosfet is a four terminal device. Mosfet would be truely "Unipolar", if the fourth terminal is available in the package. But as shown in the video, usually standalone (n/p)Mosfet chip has only three terminals. Where it is understood that, the substrate and the source terminals are internally shorted.
Now, if you connect the three terminal device in reverse way, i.e. source connected to power supply and drain to the load. Then, during the reverse power supply condition(VIN = -12V), the substrate will be biased at -12V and the drain at 0V . Hence, PN diode between drain-substrate will be forward biased and current will start flowing, i.e. no rectification.