I don't think he's doing that. At least not planning to.
That is not the kind of hedging I like in my life. I'd much prefer to be at a place where either I will make a graceful exit at 800 million and the company continues to deliver, or it never becomes worth than the 50 million, the last valuation it raised money at. (hypothetical example). If this hypothetical latter example fails, fine, nice learning experience.
If you fail after convincing someone to buy some of your company at a valuation of 50B, you're done. Facebook can never be worth less than it's been valued at, and there are plenty of similar properties that in retrospect were "bubbles". (myspace etc). This is why I wouldn't like to be in his shoes.