There is one scenario in which the 70 billion was at one time made-up money, but where it won't ever actually tank the crypto market:
1. Tether prints $1 billion with no backing.
2. Tether buys $1 billion of Bitcoin with the the fraudulent tokens.
3. Bitcoin triples in price.
4. Tether sells 1/3 of their position in exchange for hard currency, and holds that money in their reserves.
5. Tether now has 100% reserves backing the tokens they created, and is also holding $2 billion in Bitcoin. Maybe they sell the extra Bitcoin to diversify.
6. Rinse and repeat.
Under this scenario, what these guys did was to take a massive uncollateralized loan from ordinary consumers, use it to buy buy Bitcoin, and sell off the Bitcoin in order to pay back the loan. Basically an uncollateralized version of what most DeFi borrowers are doing right now, but at a massive scale.
As long as the price of Bitcoin is going up, the scheme a surefire way to make a lot of money. If the price of Bitcoin starts going down before they can cash out, then it can easily bankrupt them and crash the market—which is also a big risk with DeFi.
It could be that we are passed that point with Tether, though. With as much scrutiny as they are under right now, if Tether ever defaults, the people who run Tether are going to be wanted for criminal prosecution in dozens of countries. They will be running and hiding for the rest of their lives. I can't believe that a few more billion dollars in the bank is worth the risk—especially if they won't be able to access those billions.
If you look at the rate of issuance of Tether since June 1 of this year, it has flattened out dramatically.[0] Could it be that the people at Tether have realized that the jig is up, and are now just trying to get things in order so they can walk away clean? Maybe.
Or, it could be that they printed so much funny money last year that the amount of Bitcoin they need to sell is more than the market can accept without tanking the price of Bitcoin, given current market liquidity. Maybe they aren't generating more Tether for themselves because they figured out that whenever they try to sell the Bitcoin they need to sell in order to replenish their reserves, they crash the Bitcoin market. Maybe they're stuck.
Or it could be that Tether is fully backed, that they have never minted any Tether that didn't correspond to $$ in their bank account, and the reason that Tether's growth has slowed is only because people are now buying USDC and other stablecoins instead. (Not likely, in my opinion.)
[0] https://coinmarketcap.com/currencies/tether/