I'd guess fridges are annoying enough to be without one that many people who can afford, either through having cash or access to a credit card, would indeed be willing to pay for the second fridge while waiting a few days for the first fridge to ship back and be processed before they get the refund.
Or maybe by the time they've fully made up their mind to return the fridge, it's often after they've first decided there is indeed a better fridge out there, and already chosen where to buy it in the day or two before they start a return. In which case the time to advertise to them would be the window after purchase and before initiating a return.
Maybe... etc.
I don't know for sure, having neither worked at Amazon nor tried to market fridges. But I have seen many examples of this sort of retargeting of people who've just bought something with ads of the same type of thing, where general intuition, even that of experienced marketers yet alone normal shoppers, made it seem like a dumb idea and yet the data actually did prove it was a great use of advertising.
So maybe your gut instinct is better than the ad conversion predictions Amazon's code is making, with all their data. But I'd be surprised.