Hi Pawel, it is a really good resource. If you watch the end of the video he tests the pump successfully for an hour off the car. However, once installed in the car it leaks from the overflow pipe. The creator of the video puts this down to back pressure being present on the pump once installed. This increases the differential pressure across the seal and any small flaws in the rotating (bellows) or static (ceramic ring) parts of the seal are exposed.
This is the difficulty with these pumps and may have been one of the drivers for Bosch to develop the short version which doesn't have the seal.
He also has some comments about the quality of components in the aftermarket kits which are worth noting.