I've no idea whether they do this or not but it's easy enough to know when a tab or window is focussed or not. I have a games site and use this to pause/unpause games, music, etc.

They could hook this into Google Analytics and subtract the time you spent with that tab/window not focussed from your session duration.

I do fundamentally agree that session duration may not be the worst proxy for worthwhile content or, at least, engaging content, snd I think Google may use it as such.