What is shown in that video is assessing the
absolute minimum necessary fluid before starting the engine. It is a process only used if there was a catastrophic leak and you want to begin restoring the fluid to the proper level. To properly set the fluid level the engine needs to be on and the fluid hot.
What
@repairman54 wrote about fluid being hot is key to the actual process. The fluid in the unit needs to be hot - specifically 85-95C - and checked with the engine running. If you check at 75C and adjust fluid based on that temp, you will end up with an overfilled unit as the fluid hasn't fully expanded. If you check with the unit at 105C and drain until a drip, it will be underfilled. The biggest problem with the video above is the temp is totally unknown and the car is off. There is a Fluid Level Control Valve inside the 6T40. When you cross 60C, it closes trapping fluid inside the valve body. If that car has been driven and the unit reached say 70C, fluid is trapped in the valve body making even the minimum fluid check useless.