I don't think Musk is intentionally lying about autonomy, this is just his worldview. He truly believes that strong AI is just around the corner and it poses an existential threat to the human race, and by the same token, of course we'll have full self-driving cars as soon as just a few small kinks are ironed out. These beliefs stem from a very strong worldview, and are not attempts at stock price manipulation or some kind of investor scam. Musk is convinced that there isn't a very big difference between the human brain and the tech inside GPUs combined with some matrix multiplication and training data, and these differences, to the degree they exist, are easily bridged with a bit of software. That's a core element of his beliefs.

What is more interesting is why he hasn't learned that his worldview is so fundamentally wrong. And for that, I would say that it's hard for very rich and successful people to acknowledge they are so completely wrong about important things.