This isn't really true and the guy saying "If someone showed me a way to do it with Goldman Sachs as a counterparty, I’m in" is also misstating things, perhaps intentionally.
I'll take the long side of the Tether bet for 30% a year (I'll buy one-year Tether forwards at 70c).
I'm not as creditworthy as Goldman, but for small amounts, it's pretty close. I could collateralize the trade with my house, and in any case I don't have the kind of correlated portfolio that means if Tether goes bust, I can't pay you. But enough about me - some trader at Goldman will absolutely do the same deal! Maybe they can even do it for 72c.
There is a market-clearing price for this trade if done between two creditworthy parties. I don't know what that price is because there isn't a big public market for it, but if you shop the trade around, you'll find a price.
The people bemoaning they can't short Tether mean they can't short it at what looks like it should be the right price, say, paying 5% a year to borrow it. But that's not the right price! That's the price that already includes you taking a lot of wrong-way counterparty risk. Against Goldman, the price is 20% or 30% or something, and you can do the trade, but you don't want to.
So the fair price of one-year Tether forwards is 75c or whatever, but the spot price is clearly $1.00. How do you reconcile this in financial markets terms, that the prices don't converge? Same reason other commodity futures might have backwardation - holding Tether provides some value to its owners. Like having steel today allows you to build a skyscraper and start collecting rent, so spot steel trades higher than future steel if the construction business is good. In Tether's case, that value is the freedom to participate in other crypto trades, or perhaps to escape even riskier assets in China, or something else.