Upwork is even easier to game it than that. use a virtual machine, do the "work" there and minimize it while you play games or whatnot.
But in the end is not about true surveillance, it's about liability in case of disputes between client and freelancer. Source: I work as freelancer on Upwork for 13 years now, only hourly projects and always with their desktop app turned on.
Also it seems ridicolously easy to game, just buy a seperate upwork computer and when you go afk, just leave the right program open.