I find that answer unconvincing and there is no published data to back it up, but it is repeated constantly here.
In theory since they also have purchase data their 'purchasing intent' categories could exclude people who have subsequently purchased from $productcategory for products which are substitutes (but definitely not products which are complements/consumables/collectibles etc, because someone who has just bought guitar/fishing accessories isn't just likely to buy more guitar/fishing accessories again in future but a lot more likely to than the average person), and yes they could even make guesses about whether somebody is likely to want more than one office chair based on whether they're purchasing as a business or an individual. But in practice it's not easy when they've got a product inventory that's so enormous, complex and dubiously-labelled even the basic product search doesn't work that well, and most of the time the vendors are paying them to show the ads anyway. And still wouldn't be perfect (ironically I seriously considered buying two different vacuum cleaners for different use cases this week!)
It's surprising how bad basic targeting is, like when Facebook allowed ad targeting by sexual orientation but even dating websites spent vast sums without bothering, so its unsurprising subtle distinctions about which buyer types only need one of which office accessory are often missed.
Also sometimes there may be communication lags between retailers and ad networks. Some retailers may only upload purchase data daily, sometimes there are gaps in uploads etc.
I think most advertisers would prefer to not advertise the same item that people just bought, but it takes perfect execution both technically and from the marketers creating campaigns to pull it off cleanly. When you’re managing dozens of campaigns across half a dozen ad platforms, it’s easy to miss some of the details that matter. And while the wasted ad impressions may annoy the consumers on HN, it is likely a rounding error for advertisers and not a huge cost driver.
But it's probably true that there are a lot of individuals out there who have done this (for whatever reason), and it's the statistical likelihood of people buying a second widget that drives this.
An individual who doesn't do this, can't perceive this "fact" of statistics, which is human behavior in-aggregate form. Individuals only perceive individual behavior.
Given how widely this policy is criticized, Amazon must be very aware of it and they must continue to do it intentionally. I would trust, given the amount of money on the line, that they see evidence that tells them it's a reasonable choice.