Although I can't watch the video at work - I assume it's one of the many videos where the entire motherboard is submerged in mineral oil. Certainly that's the case, and it's why overclockers have been trying this for over a decade. It's just a problem with moving the heat away from a very concentrated area of the die. As lutorm observed, the viscosity (combined with the inability to transfer heat quickly) ends up preventing an efficient mineral oil based cooling solution.