How is advertising zero sum?

It is true that ad spends generally result in diminishing returns, however diminishing returns are far from zero sum.

In industries subject to economies of scale advertising is actually really good for economic efficiency and does not at all waste resources rather causing them to be used more effectively.

Because after the initial period of your customers not knowing about a whole category of products, you're just spending money on getting people come to you instead of to a competitor. It's zero-sum, because what you and your competitors spend on ads mostly cancel out - you're competing for a limited, finite pool of customers.

It's zero sum for the advertisers - for each ad spot, there can only be one ad, so if I win the auction, you lose. Of course, since it's an auction, the winner will be spending increasing amounts of money on it, with no real upside (still the same amount of ads displayed).

It's all very good money for the ad platform/brokers.