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.