I wonder how much of the purported "success" of retargeting is just mis-assigned credit? You research a camera, but it takes you some time to make a decision. In the meantime, you get blasted with camera ads, which you ignore. Then you actually make your purchase. Even though the ads had no effect on you, there's a fairly high chance you still purchased an advertised model at a vendor that targeted you with an ad for the camera you bought (there are only so many of each). The advertisers may claim credit for that sale, even though in reality they deserve done.

Even more egregiously, maybe sometimes the advertiser's metrics have some reverse-causality: you buy the camera, then they show you the ads for it; but something in their metrics isn't properly modeling the sequence of events (e.g. they correlate retargeted camera ads shown per week with camera sales per week). Then they mistakenly take credit for a sale that happened before the ad was shown.

That's not to say such mistakes always happen, but I wouldn't be surprised they didn't happen fairly frequently. Given that measurement is hard and both adtech and advertisers are motivated to present success stories, and may not be too motivated to second-guess positive-sounding numbers.