Which means it's paper wealth. Which means there's no way he could turn any significant fraction of that into USD or any other form of value without obliterating the value.
https://www.bloomberg.com/billionaires/profiles/jeffrey-p-be...
If the economy tanks, it makes Amazon's review data and free shipping even more valuable. If I'm strapped for cash I'm less likely to walk into a retailer and pay top dollar for something with no reviews.
I prefer the Cliff Asness bubble definition, "the term bubble should indicate a price that no reasonable future outcome can justify."
Looking at a lot of those markets, it is definitely possible to see future outcomes that make the current prices justified.
The stock market? After the tax cuts, Shiller P/E is on the low side of the 20s now. With another few years of solid earnings growth, there's definitely a possibility of justifying these prices.
The bond market? Look at 90s Japan. That could easily justify those possibilities.
Art and real estate? We have more rich people who want to have their capital flee countries like Russia/China than ever. That is a trend likely to continue.
So I'd refrain from using "bubble". Valuations are stretched, but not absurdly so.