My guess is that there is every possible combination of items that is added to the cart, and then the API response returns what items are unavailable/out of stock.
Honestly, I'm not in love with this post for HN. It seems like something done and communicated for twitter clout and is devoid of interesting details.
Because it's a meaningless number. If he is actually placing an order for that amount every minute, it would be a useful number (and he would be an asshole for making so many fake orders and filling their system with garbage metrics). If he placed ice-creams in the cart for 8000 stores, then he should give that number. The dollar value is a completely pointless number in this case, and was done purely for clickbait.
Not original commenter but I think jackric already alluded to the increased cynicism that develops from clickbait. Personally, when I see so many ppl giving up honest and nuanced statements/opinions for hyperbole in order to gain views/likes, it's disappointing.
Raises the noise in the signal-to-noise ratio of a browsing experience. I may pass over reading a useful truthful article because I'm trained to dismiss its title as misleading.