Good questions. Some comments:
1) For the Standard Range Model 3, the battery pack only needs about 55kWh, so cells with a lower volumetric energy density (due to a different chemistry) can be used.
2) It appears that Tesla has decided that since they are cell supply constrained, they can thus use different types of cells with a lower energy density for their SR variants without compromising the vehicle's performance.
3) This in turn allows Tesla not only to build more SR cars, but also,
4) frees up their proprietary cells for use in LR (and S/X) cars, as well as
5) denying the lower density cells to their competition.
There are even indications that Tesla has created a variant of their battery pack that can take prismatic cells (so a whole different form factor), presumably while maintaining their ability to use their liquid cooling to control the temperature of these cells in the pack. If true, this means Tesla can extend the range of possible cell suppliers to those who have capacity for making prismatic cells.