One possible reason is it's very hard to prove the market value of something that you never sold.

You can say that your product is worth $1bn until you're blue in the face, but there won't be any evidence of that without any buyers.

I think it's more basic than that. The company has no BATNA (Best Alternative To Negotiated Agreement).

Could he denounce his US citizenship and move to, say, Moscow or China?

If he were willing to put himself in the a position where the US government could claim he had broken the law sure he could do whatever he wants. That would arguably make his bargaining position worse though because then he has the US government after him for what they will likely frame as espionage and he would be dependent on the good will of some other government which effectively puts him in an Assange Snowden style predicament.