It's a German thing, seen the same in VWs and Porsches of the era
The idea being that (with such a quiet engine) you don't try to start a running engine. Lesser cars would give you that unpleasant scraping sound of the stater gear trying to mesh with a turning flywheel.
I am trying to teach my teenage kids that they need to LISTEN to the engine starting before then releasing the key, unlike our modern cars where it is all done by the computer, you can flick the key (my son) or just hold hit there for 30 sec (my daughter). Old cars are just sooooo complicated - whatever..