But it didn't actually enable short selling, it created a USD-settled market for betting on the price of Bitcoin, which is a very different thing. The article is simply wrong.
CME and CBOE volume paled (and still pales) in comparison to Bitmex, where traders can and did short Bitcoin well before the regulated markets launched.
For every short, there's a long.
Long, but at what price?
The buyer pays the exact same price as matched to the seller, plus some tiny fees for the exchange and clearing.