A "stablecoin", as far as I can tell, is just a cryptocurrency backed by an unenforceable promise by some entity that they will always buy the currency at a fix

A "stablecoin", as far as I can tell, is just a cryptocurrency backed by an unenforceable promise by some entity that they will always buy the currency at a fixed price. If SG says "April Fool's!" in two weeks and declines to actually buy back the coin anymore, what would be the cost to them? I doubt anyone would withdraw their real funds just because the bank abandoned its crypto experiment. The article talks about a "secure legal structure", but at the end of the day, can any holder of the coin sue SG for its promised value? Without that, "secure" is meaningless.