Certainly marketing is part of it. What Herse - and the other top constructeurs - did was to create a bicycle that was a fully integrated whole dedicated to a specific purpose (i.e. not a frame with a bunch of bolted on parts that could be re-rigged for different uses). No one else was doing that to the same degree, aside from those few top-end French builders. One can argue the aesthetics of a Herse (or any other bike of course), but the details of construction are in many cases very well thought out and executed. Some of the parts were made in-house and designed for the specific bike model in question. Some also sweat the fine finishing stuff (i.e. lots of filing) but many do not; their "genius" is in their overall design and the degree to which they are precisely adapted to their owner and intended use.