By definition, you can't have a coin be stable while at the same time not having a centralized instance that keep it "stable". Those are oxymorons. It's one, or the other.
We can't just repeat any buzzword without thinking about what exactly does it actually mean in practice.
Decentralised stablecoins, such as Dai, do exist, and the price of Dai does manage to maintain its peg pretty well. Not all decentralised stablecoins have done as well (most notably TerraUSD) though, it's definitely hard to design an algorithmic stablecoin that maintains its peg.
I think that commenter means it's logically impossible. In what sense is DAI "decentralized" if its value is set by the same centralized group that sets the value of a dollar?
Mind sharing where you got that definition from?
Though bitcoin volatility has been going down along the years, it might be quite stable after 10 years or so.