That's not really true. On a literal level, yes, the specific dollar spent in one place isn't spent somewhere else, but the increased attendance at one park doesn't have to be cannibalized from another. If that were the case, it wouldn't make sense for WDW to have four parks. Some people, of course, will just say they have X days in Orlando and each day they choose to be at one resort is a loss for another. But new attractions also drive increased Orlando-area tourism which ultimately benefits everyone. For example, a family that likes both resorts may have not been thinking of coming to WDW next year, but who comes to Orlando to visit something new at Universal, may decide to put in a couple of days at their WDW favorites. More people visiting Orlando for whatever reason puts more money in local pockets, some of which will get spent at WDW. And so on. Economics is not a zero-sum game.
In short, Universal upping their game is both a benefit and a concern for WDW, and the long-term impact isn't easy to predict.