The reason why this is possible is simple.
The mineral oil is not electrically conductive.
The first time I saw something like this was on 'Tomorrows World' about 15 years ago when they sunk a working CRT television in a tub full of non conductive liquid. And it still worked.
Notice in the video that a finger is added to the mix when testing the PSU? Tap water can be filtered to remove the electrically conductive components from it that makes this possible.
In fact id say that the some mineral oil is less conductive than air!
Also when you turn it off it will turn off. When you turn it on it will turn on. This is no different to 'air submission'.