I think too many people equate fiction and lies. Stock prices and market caps are not complete lies (the company is still worth something) but they are fictions. I like to think of fiction as "a lie that tells a truth that cannot be told any other way" (adapted from Albert Camus).

Market caps and startup valuations are fictions. That doesn't mean that they don't say something important - they convey information about who has market power, political power, technological progress, etc. All of those things are worth a lot. Fictions can affect reality. They are frequently more powerful than truths. Theranos, Nikola, and Tesla have all moved billions of dollars of real money on the back of fictional product descriptions.

Nobody said it’s a fiction. What is a fiction is pretending the current share price * number of shares is what the value is. That’s completely detached from how the stock market works.