That's one way the economy is not zero-sum, but not the most important way. The most basic example of positive-sum economic activity is that if I have something that is worthless to me but valuable to you, and you have something that is worthless to you but valuable to me, we can create value simply by exchanging these two items. No goods were created or destroyed, yet both of us received more value than we lost from our own points of view, and thus net economic value was created by the exchange.

Regarding the real-estate example, the real estate may be zero-sum but net economic value still increases when a piece of land changes hands from someone who values the property less to someone who values the property more—which is the default expectation underlying any voluntary trade.