market cap is a meaningless number
i can create an alt tomorrow with 5trill coins.. get just 1 coin on an exchange and sell it to myself for $1.. and instantly my alt is market capped at $5trillion...
there are NOT $25billion fiat sitting around in bank accounts ear-marked to back the entire cryptocoin market.
anyone can make the market cap spiral up to trillions just by using small numbers.
this is partially true, there is no way to figure the exact amount of the money sitting in all the exchange, maybe they are not 16B like the bitcoin marketcap, but they are not only millions either
last time i've checked all exchange combined are sitting on a large amount of bitcoin, more than millions in number, so the real marketcap is not so far away from the one reported in coinmarketcap