Earlier quoted context omitted.
If the US or EU (etc.) valued their goods and services in BTC, banked in BTC, built monetary and foreign policy around BTC then you would be correct. It is all meaningless, and that's the definition of fiat. What distinguishes one fiat from another is "do important people believe in it". There may come a day when that's true for cryptocurrencies, but that day is not today. Further, I doubt it ever will come, because…
I agree with the argument that it doesn’t make sense for the United States. Other countries using USD already don’t have control over monetary policy.
But for those countries, it really seems like choosing Diem is the worst of both worlds. You're still pinned to the Dollar, you're still at the whim of unaccountable controlling stakeholders, and you have all the downsides of cryptocurrency on top of it. I struggle to think of a group outside of Facebook that Diem is at all good for, honestly.