And the author figures out it is all about liquidity, which it is. The reason you might take a Euro for something is that you can give it to your bank and they can convert it into something you can use for more things (dollars, rupees what ever).
I'm really curious though about what happens when someone walks up to Coinbase (as an example) and says "I've got 150,000BTC, please convert that into dollars for me at the market rate. What? No worries I'll just wait here. You can wire the dollars to this account here ..."
Let's assume that BTC were trading at $600 each, that is $90M dollars. Where does Coinbase get the $90M to cover their side of the transaction? Since nobody "bought" the BTC from Coinbase (perhaps some was bought there, but perhaps someone had just stolen it from the FBI or something) the counter party to that transaction (the person who gave dollars for BTC or gave watt-hours for BTC) their contributed value isn't at Coinbase, it is spread across perhaps a half dozen exchanges.
How does that even work at scale?