[MarkSweep]

It makes more sense if you look at these rates:

https://blockfi.com/rates/

To qualify to borrow at 4.5%, you have to have a loan to value ratio of 20%. If I understand correctly, that means you have to deposit 5x crypt than the value of the loan.

On the savings side, the rates vary. If you deposit BTC, you earn 5% for the first 0.5 coins, 2% for the next 19.5 coins, and 0.5% for the rest.

Since the loans are secured, if the value of bitcoin does not move too much they can cover defaults by liquidating the collateral. Given the volatility of crypto currencies though, I still assume they will blow up at some point in the future.

[Animats]

They claim that if you deposit USDT, they pay 8.5%/year.

They're effectively speculating that Tether will crash, and they get to pay you back with cheap Tethers.