The question is whether the whole mobile game industry makes more money by advertising, or whether those games that advertise just make more money at the expense of others.
Imagine that in a world without ads, game revenue is: Game A: 600, Game B: 600, Game C: 600
But then A spends 300 on ads: Game A: 1200 (-300), Game B: 300, Game C: 300
In this case, game A "spent money to make money", but the overall industry's revenue was the same, and part of the profits now went to ads.
And it makes sense: advertising doesn't give consumers more money to spend, so the size of the pie is fixed, all they can do it redistribute it, extracting a slice in the process.