This is per Tesla's current claim that their 20 GWh of annual, automotive battery production is equal to (or more than) the combined annual, automotive battery production of all other automakers, i.e. a current, total annual production of at most 40 GWh.
I think that claim is dubious. According to EV Sales 145.000 Plugins were sold in July. Substract ~20.000 Teslas and you are left with 20 GWh of batteries for the other 125.000 Plugins. That seems to little, even if you discount the average pack size for Hybrids and leave out busses and trucks completely.
So for VW (which has no history of vertical integration) to simply assume that they can buy their 50+ GWh batteries annually is in my view overly optimistic.
Why? I'm pretty sure, if i go to CATL tomorrow and say: "Here you have $5 billion and there are supply contracts for the necessary primary resources. Could you please build a factory right over there and deliver me x GWh of battery cells four years from now." They would probably say something like "Hmm...kay." and get to work.
Bear in mind that Tesla claims to have a roadmap for bringing their GF1 up to about 100 GWh annually. With their ongoing progress, Tesla surely already has contracts with suppliers of raw materials also for the future. So it is not obvious that even the supply of raw materials for the batteries VW will need is available (at prices that will allow for a profit).
Well, if i'd believe in Teslas roadmap, i certainly wouldn't be in the bear camp. And i do not see a reason, why Tesla would be able to secure that amount of resources given their limited financial resources and VW couldn't. Which of course only is a problem, if the necessary resources are as limited as you say. About a year ago, Musk said that Tesla was aiming for 1 million cars by 2020. That seems to be delayed by 2-3 years, until the china plant is up and running at full capacity. So which one is it? Did they sign to many supply contracts and sit on to many resources? Or didn't they and the race for those resources is still open?