What I think distinguishes Squid Game over The Platform most was that Squid Game directly portrays (and thus critiques) the society outside the "game" whereas The Platform is a self-contained allegory.
(Spoilers galore)
In both participants sign up without initially knowing what they're signing up for and in both there are volunteers who join despite understanding the violence and death, thinking they can "win" (i.e. the returning participants in squid game and the bureaucrat in The Platform). But while in the latter the only reason anyone would subject themselves to the system is suggested to be extreme naivety, in the former we are shown the desperation and hopelessness the participants face outside the game that drives them to such an extreme decision simply for the promise of a chance at getting out of their situation.
Both systems are set up to disrupt solidarity and cooperation (where The Platform even goes so far as to claim the exact opposite). Both pieces of media demonstrate those systems succeeding but end with an act of self-sacrifice.
But Squid Game gives us a motivation for participating in such a suicidal system and the most horrifying aspect of it is that it is not some kind of sci-fi dystopia but what TV Tropes would call "20 seconds into the future": a setting that feels mildly detached from our present but you could easily imagine being set in the present day in a Western country not unlike your own. Yes, the squid game itself feels excessive and has sci-fi or at least fantastic undertones, but the world outside it feels unsettlingly familiar.
