They could guarantee it by making a legal contract to accept the stable coin for a value of $1 always. I am not saying they will, I am saying they COULD. Like they can create 100,000 stable coins. And then accept them for services and price them at $1. Anyone who has stable coins that wants to sell them, can receive $1 guaranteed via a legal document. Or Facebook could just say its $1 of value and not redeemable via cash.
When people use the stable coin for services, facebook can then sell those coins to others who want to purchase for a standard $1.
They could make it decentralised if they wanted, but who knows what will happen until it does.
If that's the case that they're going to guarantee it's backing via legal contracts, then how is it in any way "decentralized"? That's the exact opposite of "decentralized".