Those 1.4 millions drivers aren't all driving at the same time. They would only need a fraction of that many cars and they could be built over an extended period of time.
or areas/weather conditions which they know are difficult for their autonomous cars to navigate.
If they bought Lyft, they wouldn't have to do that all at once; they could just insert self-driving cars as they were available into the pool.
Optimistically with economies of scale you can get that down, but even without it's within the realm of possibility if they've got the technology working.