It isn't like VW is much behind Tesla in EV sales: 171k BEV plus 171k PHEV vs. 386k for Tesla in H1 2021, and VW had faster EV growth than Tesla compared to last year.
I suspect this is more about interoperability, e.g. a charging station deal or something similar.
VW: https://insideevs.com/news/521666/volkswagen-group-sales-plu...
Tesla: https://www.statista.com/statistics/502208/tesla-quarterly-v...