My guess is they own accounts which show the ads, so they get paid for showing ads. This is similar how those "get free money" apps work, they own Ad account and get paid when you view/click that ad.

Another theory is since lots free games have where you can earn in-game currency for viewing ads, the bot farms might be abusing this method.

I host a page, put google ads on it, hire you to generate bot clicks on said ads. Google pays me, I pay you. Some poor sucker somewhere pays google.

exactly! the article fails to provide any non-obvious behavior of Google Ads