The value of X, for any X at all, is what someone else is willing to pay for it. That is how markets work. Any kind of market. And it works for everything from pricing apples to pieces of art to companies.
If people are willing to pay $1 billion for your company, it is a billion dollar company. If nobody is willing to pay you a dime for it, it is worth nothing. Your revenue is an input factor into what people are willing to pay, but doesn't determine the price.
If you study financial theory, the theoretically correct price for a company is the "expected present value of future revenue". Meaning that if you look at all future revenue that it should ever make, divide that revenue by a discount factor for the fact that a dollar tomorrow is not worth a dollar today (and further discounts for risk), that number should be the present value of the company.
So a company with little revenue and good growth prospects may be worth much more than a company with great revenue which is going off of a cliff. It is worth this both in practice and in theory. And anyone who says otherwise simply doesn't understand how to value companies.
Note that in practice, in illiquid markets (which stock in privately held companies always is) the variance of market value from the theoretical relationship becomes wider. We still price companies based on what someone, somewhere, was willing to invest in it. We do that because someone educated people, who is paid to get this right, with possession of more facts than we probably are, decided that this was a reasonable price after doing research. They may be wrong, but the last price paid is the best market indicator of the value of the company.