I would not trust it at all

see https://www.reddit.com/r/Piracy/comments/dzl651/accuracy_of_...

So I think this is just linking IP addresses to the things they've torrented. Which seems not terribly difficult, but also prone to some obvious errors (since residential IP addresses usually aren't static).

But also, mentioned in the comments of that Reddit -- the site they are talking about:

iknowhatyoudownload.com

is different from the one linked here

iknowwhatyoudownload.com

I didn't click on the one described on Reddit, because it seems likely to be a sketchy typosquatter based on the comments there.

yes, just useless (and a lot of ads)

They don't, lol. Got a few ad impressions off me, though.

Having reviewed the list I can confirm what you expected - they don't

I am sitting in a hotel and apparently someone downloaded a Russian version of Photoshop :D

So this is about torrents. I never got into them, but isn't the whole point of torrents that it is known what people downloaded? That's how they know what you can seed, right?