I agree that this particular video does not show the whole process very well. The hydraulic tensioner does not apply torque to the nut, it applies a tension (stretch) to the bolt. Then the nut is tightened manually.
The goal is to provide clamp force on the joint by stretching the bolt.
It the usual bolted application, this is done by tightening the nut, which acts through the ramp angle of the thread to stretch the bolt. There’s a lot of things that can affect how that torque translates to tension in the joint. For example, friction between the mating surfaces.
Hydraulic tensioners pull directly on the end of the bolt to create a certain amount of tension. Then the nut is tightened to maintain this tension. There’s still some variation in the results, but much less as compared to a simple torque tightening.
There are lots of other methods to tighten a bolt/nut connection: torque-angle, turn of the nut, and now exotic computer-controlled methods that monitor the joint as the bolt is being tightened. But this is getting a bit off-topic.
Here’s a video that shows the two methods of tightening:
