Do you think they could have possibly had the cars delivered to Switzerland and had them drive to Bavaria from there? If I was a field producer, I would not want to send racers into Munich unnecessarily, far too many tourists...
Anything is possible. It would all come down to how direct the route from Stuttgart to Interlaken is for a truck transporter (ie main highways as opposed to windy alpine roads). I'll go look at a map now.*
Because in Germany you have to reregister your car if you move to a new district, so those cars had
to originate in Stuttgart. Another alternative to Munich would be taking the train to Zurich and self driving from there. At airports the racers are quite obvious, as there is a lot of hanging around and there are a large number of travellers (all blogging and with camera handy), but in past races, other forms of public transport such as trains and buses have had less reliable race sightings, and so a Munich train station sighting could still have easily escaped our attention.
*ETA according to Google it is nearly 4 hrs drive from Stuttgart to Interlaken, but just over 2 hrs to either Munich or Zurich, so either option would be feasible.