Neither company has pure motives here. Apple just so happens to take a 30% cut of in-app purchases, so they stand to benefit immensely from a change to that model. They also have their own ad platform that will benefit from this.
Both sides are concealing their selfish motivations under the guise of altruism.
Apple’s search ads will indeed benefit greatly from this change. When an app developer runs an ad on Facebook, and a user converts and buys the app, attributing that conversion requires “cross app tracking” that will be banned by this update. On the other hand, if the developer runs the ad though App Store search, Apple will be happy to attribute the sale to get paid. One could reasonably guess that the real purpose of this change is to take that market from Facebook.
Both sides are concealing their selfish motivations under the guise of altruism.