What seems key to me is that Tesla is the only major car company committed to equipping every single vehicle it manufactures with FSD capability.
GM, Honda, Mercedes, etc. may have the capability to manufacture 400k+ FSD capable cars per year but they are not currently doing so and will take years to ramp (3 years from P3dStealth seems like a decent guess, if they were fully committed).
By subsidizing/forcing the latest FSD tech into even their lowest price base model they will end up with a larger FSD fleet than anyone else unless the current hardware is somehow not capable.