You probably need to see the speaker notes (or listen to the presentation) basically, the battery pack is a huge chunk of cost in the car, this is especially true in lower cost cars, and the markup of the materials as they go through the process which quite high (basically that is code for 'the battery makers are making huge margins on batteries). Without a battery competitor to try to squeeze margins, you are forced to either do your own batteries or cede the margin to the battery maker.
By putting this out there you end up effectively negotiating with Panasonic (the biggest supplier) to either take lower margins or lose their biggest customer. In order to make that sort of threat you have to credibly be able to build the capability. So Panasonic (and potentially other manufacturers that see the sorts of margins they get) see this and react. One hopes they react by lowering their prices (taking less margin) or by new companies entering the field which are structured to operate on lower margins.
The overall effect is the same, Tesla negotiates a better price on a pricey chunk of their cars.
By putting this out there you end up effectively negotiating with Panasonic (the biggest supplier) to either take lower margins or lose their biggest customer. In order to make that sort of threat you have to credibly be able to build the capability. So Panasonic (and potentially other manufacturers that see the sorts of margins they get) see this and react. One hopes they react by lowering their prices (taking less margin) or by new companies entering the field which are structured to operate on lower margins.
The overall effect is the same, Tesla negotiates a better price on a pricey chunk of their cars.