I think a reasonable metric would be "you have 2 years to sell your assets and convert to cash (or something super liquid like treasuries)". This accounts for people who hold things like farm land or real estate that can't just be liquidated tomorrow. By this Metric, the values for many of the billionaires out there are real. Bill Gates, for example, hardly owns a significant stake in Microsoft anymore, but he does have lots of illiquid assets like farmland and real estate. Brin/Page/Bezos still have significant stakes in their companies, but the companies stand on their own at this point and I doubt that shares would go down a ton if any of them liquidated over the course of a year. Zuckerberg probably can't sell his meta shares without a reasonable decrease in value, so there'd be some haircut, but it's still an incredibly valuable company without him.

But Musk is at a different level. I can't imagine him selling his stakes in his companies without the stocks plummeting more than 50%.

A private company owner needs to sell his company

A high net worth investor in a fund will have signed up to maximum redemptions and may be locks out during a crisis

Most assets have varying liquidity and you can only surely judge net worth as its present book value

No need to sell anything.

Maybe such invisible "paper money" isn't such a bad thing if he can go shopping with it.