Considering $10-20bn in liquid assets might've had to be reassigned quickly and a lot of people who understood the fundamentals of luna probably converted their ust and sold each for a dollar (and dumped their luna too) - but I'm not sure if this actually happened much or how much ust there were before the crash.
You're mistaken about one thing.
You assume that there were 20 billion behind Luna when there weren't, the price tag on them was 3.5 billion but ended with just 80k
BTC BTC being liquidated between 40k and 30k, so far less than that. Second, there was no money in LUNA, the money invested that crated that artificial market cap was not in the pockets of the holders of LUNA, they went in the pockets of the ones selling the coins!
Just as when you buy a car worth 50k, the money is not in the truck of your car but is in the pocket of the dealer.
So, the amount of money that was actually taken out of LUNA (by tricking some morons to pour cash into this scheme) might be far, far less than that and it might be in the range of a few tens or best case scenario hundred of millions.
Before the crash, UST had a marketcap of around $18.5b, and in just around 6 days it's down to around $1.4b. All that sold UST was mostly sold for Curve-supported stablecoins like DAI/USDT/USDC as they had the most on-chain liquidity.
The price on the 9th was 1$, 6 days later it was less than 20 cents, most of the UST was held in Anchor by investors, and most of those didn't even have the time to react or a way to get out of this and exchange for any other coins, remember that UST was sold by Terra to finance their Bitcoin buying plan, so again, highly likely that there are millions of USDT that are still being held by their owners who just see 1$ drop to 1 cent, no value was carried over whatsoever.