My hypothesis is that Tesla "licenses" the DBE process to Panasonic, since Panasonic has been a good partner over the years, and Panasonic still has good supplier relationships (like Sumitomo) that supply the other components of the cell. This would help the mission by having a trusted partner that can expand the battery supply with Panasonic's customers, without impacting Tesla's supply.
This would accelerate the mission (by forcing the cost of batteries below the $100/kwh point, making BEV's cheaper to build than ICE's) and remove the ramp-up lag that would be involved with vertically integrating the cell production process (from the gigafactory investor day, we learned that the Panasonic cell production machine took up an entire room).
Building the cells themselves would require developing their own cell production machines and setting up supplier agreements with raw material suppliers and/or specialty chemical suppliers. It's not impossible, but it is an unnecessary burden unless Panasonic is unable to integrate the DBE process. Much faster to just have Panasonic license the tech and sell back the cheaper cells.
