Data enrichment brokers cross index purchase times with other data from legal factory installed spyware found in cash registers, analytics tools in proprietary software you already have on your computer, etc etc.
Every time you buy an over the counter medication at the pharmacy with a credit card, the data brokers know by combining information sources, and sell it to insurance companies.
But it still doesn't apply here. At most they could cross-reference it with a public Steam profile that has the game listed as owned after the purchase (actually, I doubt they would have the account ID to prove it for sure, but with enough purchases they could likely figure it out), but by then you've already sort of revealed to the world you bought whatever embarrassing title yourself.
No Steam API access or public data needed. Major game studios include analytics suites (bspyware) which also tend to collect information on what other software you have installed, can harvest data from Steam client side, etc etc.
Hmm, unfortunately, the URL is broken. "Safari can't open the page because the address is invalid." Couldn't get it to show in the internet archive, either. _puts on tinfoil hat._
Every time you buy an over the counter medication at the pharmacy with a credit card, the data brokers know by combining information sources, and sell it to insurance companies.