Well now you're just quibbling about the amount; it seems you do agree that SpaceX can be valued. You just don't agree with the <1 multiple the GP suggested. So ok, how about a 4x multiple? 8x? Whatever! Private companies can be valued; it's done every day. SpaceX is not an exception here.

Musk can say he's going to colonize Mars or mine asteroids, but the markets are perfectly capable of deciding on how likely (or unlikely) it is that he'll succeed, and price accordingly.

Colonizing Mars is a money-loser. It could completely tank the company. What business model involves colonizing Mars and then making a profit off that?

Mining asteroids is something we're so far away from that it's not worth baking in at that point. With current technology, it's far, far, far cheaper just to mine on Earth, even for things that are relatively rare.