Hi this is the author of the article. You're just making the exact mistake this article is about: you assume people who click on your ad and then buy something, bought something BECAUSE of your ad.

From the article: "Suppose Luigi’s Pizzeria hires three teenagers to hand out coupons to passersby. After a few weeks of flyering, one of the three turns out to be a marketing genius. Customers keep showing up with coupons distributed by this particular kid. The other two can’t make any sense of it: how does he do it? When they ask him, he explains: "I stand in the waiting area of the pizzeria."

It’s plain to see that junior’s no marketing whiz. Pizzerias do not attract more customers by giving coupons to people already planning to order a quattro stagioni five minutes from now."

It's the same in online marketing, you often target people who are searching for say a pair of shoes. People who are searching for shoes have way higher baseline probabilities of ending up buying a pair of Nikes, whether you show them a Nike ad or not. So if you do not correct for this 'selection bias', you have no idea what your ad did.

The research I describe in the article clearly shows (and please look it up yourself, the links are all there) that selection bias is HUGE, and that it's hard to know ROI, because true advertising effects are tiny if you measure them in an experiment.