The valuation is rubbish because it comes from multiplying together two numbers that do not mean what people assume they mean. Uber, like substantially all privately-funded corporations, has issued multiple classes of stock and probably convertible debt. The investors were issued securities that do not resemble common stock, so the standard practice of taking the number of dollars invested and multiplying it by the inverse of the ownership share they received is not appropriate. That method works only if all the shares outstanding are of a single class, and are devoid of preference, attached options, warrants, participation rights, etc. Which I can pretty well guarantee you is not the case.

Unless you have Uber's cap table in front of you, you cannot figure its valuation. The journalist certainly did not (and I don't either). The most we can say is that $50b is a ceiling on the company's valuation; in reality it is far less.

Many, many articles have been posted here explaining this in greater detail. Please read them.