Suppose BTC goes up by $1. Then, the long position (owned BTC) is now worth $1 more. But, the short position is worth $1 less by definition. So, the overall portfolio is worth $100 + $1 (gain from long) - $1 (loss from short) = $100. The value of the overall portfolio has not changed, it's still worth $100, even though the price of BTC has changed.