Knowing that you bought a mattress is useful for the ML to find the mattress-purchase-likely cohort. Filtering those people out just saves the advertisers money. Hence the grift.
I'm not saying it's likely, but there are scenarios.
I did this. It’s because I’m incompetent. But I bought stuff for each room one at a time. Use an ad blocker, so no clue if it caught.
Google Ads specifically has a feature to exclude targeting of customer's you've said have made purchases, ie. only target net new customers.
The platforms aren't as dumb as the post author thinks they are.
People who have recently bought a mattress will see a lot of ads for mattresses.
Maybe the platforms are not that dumb, it is the mattress seller that doesn't properly make use of that feature, or there could be other reasons why they insist on suggesting over and over again a mattress.
The experience described has happened to everyone for any (not very often replaced) item bought online, be it a printer, a wash machine or whatever, so if the platform is not dumb, it lacks data and the sellers are dumb, in any case the final effect of this "targeted" ads is 0 added sales.
Think of it — some people have a second home. Or a short term rental. Or a guest bedroom.