yeah exactly. if you're cheating online at a high-level, would this not be the exact first thing you'd think about dealing with? chess.com can have all the crazy algorithms in the world to try and figure out if you're playing like a human, but if you're tabbing out every time your opponent plays a move, you're giving the game away in move 1 (or - to take a more literal approach - whatever move takes you out of your opening repertoire...)

and yeah you could argue that if you're not into web dev it's not hyper obvious that websites are able to know when you lose focus on them, but I'd argue that it is fairly obvious. there are indications that should make it clear. facebook flashing notifications in their tab title until you click on them, to take a simple example.