Strange concept, considering there is no set metric for "valuation" for a private company... so who's in charge of this mythical list of which SpaceX somehow belongs?
Typically private companies are ranked on their revenues, not their market cap - since such a metric is sketchy at best.
In 2006 Wikipedia took a shot at it and they pegged number 1 at 1.56 Trillion dollars. #1 was 140 billion. 11 years later I'm sure the entire list has moved way higher than that (after all, the #1 public company went from a valuation of $447M in 2006 [Exxon] to $617M in 2017 [Apple]). So I'm not sure how a mere $21B puts SpaceX "among the highest valued private companies in the world".
I realize these are our heroes, but we don't have to just make stuff up. They can stand on their own legs and achievements.
In the real world, you are absolutely correct. In Wall Street analysts and speculative "news" articles world then you can make sketchy guesswork assumptions. Luckily, it is a private company so sketchy guesswork has little effect on the more critical aspect of getting real things done. Happily, SpaceX is doing very well in the real world; no matter their real world or made up world valuation.