Earlier quoted context omitted.
The reporting is skipping some steps. You'd buy UST then deposit it into a "separate" bankish thing called Anchor which would pay you 20% APR. The interest was supposed to come from Anchor loaning out your UST at a higher rate. This would be sustainable in theory, but they were actually paying interest out of their marketing budget to bootstrap the Terra/Luna/Anchor "ecosystem".
So I take my coin, pegged to the dollar, and lend it Anchor for a 20% return. Anchor lends it out to someone else at >20% APR because that other person is… brain dead? Who would accept that? But again, where do the new Terra coins come from? If Anchor pays me in more stable coins, they had to buy USD to be allowed to mint them right? Does that mean I bought Terra with USD to allow Anchor (which is Terra?) to sell my…
Collect many of those short-term high-interest loans into a single bucket, and you have a longer-term high interest facility. Like being an LP in a credit fund. So there is a fairly high fundamental yield. Not 20%, but maybe 6%.
Second, as the GP mentions, they were also using their marketing budget to pay an unsustainably high yield to excite and entice investors. So on to the 6%, add another 4% of VC money.
Finally, they were underestimating the risk of the underlying loans defaulting, thereby causing them to overestimate the APY on the pool. That brought them to a place where they thought they could pay 20% instead of 10%.
There might be more to it than that but that's what I understand.