Where do you see that number? And what makes you think that what you see actually reveals what you think? If we are talking about currency or stock markets, the other side of the trade is an exchange itself, that's why your logic won't work with them ("for every short position taken, there's someone on the other side of the trade taking a long position").

If you actually mean crypto exchanges like Bitfinex where you can sell borrowed coins, this is what it is. The number of borrowed and then sold coins shows the market sentiment (provided you can actually know this number), and there is no other side in the sense you mean it. Someone has obviously lent these coins but that doesn't in the least mean that he expects the price to rise. So your reasoning fails here too. If anything, it is not a bullish indicator.