Thanks for your insights!
I tested this yesterday: After re-tightening my chain according to the manual I measured the play like the guy in the video in front of the chain guard.
But contrary to what HoodRichOG and smithy assumed, I ended up with significantly
more play than 30 mm there, i.e. using the video-method would cause an over-tightened chain.
Surely the video-method is easier, but from what I know about chain drives I would rather err on the side of slack. Erring on the tighter side could lead to higher wear of the gearbox bearings which would be a major issue if it has to be changed.
BTW: Another mess-up in the video: Torque for the axle nut is said to be 80 Nm.
Manual of my '22 Ribelle says 70 Nm with a tolerance of 5%.
I am aware that chain tightening is not an exact science
But from a premium manufacturer I expect that they are sticking to their own rules or give at least an explanation when deviating from it.