There was no reason to believe that any car manufacturer would OEM FSD at this point. It's not even a reliable level 2 system and they would need Tesla computers, cameras, and to feed data back to Tesla. Tesla would have to make adjustments for their brakes, size, control, etc.
Many said that was BS over a year ago. Solved FSD, sure, but not the Beta product we have now that 5-10% (optimistically) of the time drivers are forced to take over. My car uploads as many as 6 gb in a night after long drives. That's not feasible for most manufacturers.