They do exist and are still redeemable for Euros at fixed rate. Little weird but germany is a big country that wants people to trust it, so 20 years after Euro was adopted it's still worth 1.95 deutchmarks.
Many of the same people who buy the Bitcoin-as-global-reserve-currency scenario also believe the Euro is going to collapse. It has known problems as a monetary union without a fiscal union, which lead to periodic bailout crises.
Here "deutsche mark" (and lira, which is another dead currency) are stand-ins for whatever national currency emerges after a Euro collapse. It may not be called that, but the point is that there will be some German national currency that settles up into Bitcoin in the international markets.