We don’t explicitly say in the video, but you can tell that we had power disconnected (at the breaker for the dishwasher) by two things. First, we’re doing an ohms check, which is always done on a dead circuit. And second, you can see that in order to access that P5 connector, you have to have the board pulled out from the machine — something you wouldn’t do with power still applied.
Live testing of a fuse is certainly possible, but it wasn’t the most convenient thing to do in this case. If you were to perform a live test, you would check the voltage drop across that fuse instead of the ohms.