I don't think people assume there is literally zero signal. I think their intuition is that the repurchase rate is low. Low enough that that if the advertiser knew the purchase was made, then the advertiser would serve something else with a higher predicted chance of getting a click.

It's easy to assign a straw-man a belief there is zero signal and talk down to it. The more nuanced idea that is less obvious goes like this: advertising is such a boondoggle that the weak signal they intuit actually does represent a higher click-through than the rest of their ad inventory.