Comment by ains

For what it's worth the claim is "2x the national average in interest", the national average in interest is incredibly low as many people have their money in checking/savings accounts at large banks with near 0% interest rates. It's very possible to give people twice the national average and still make money on the spread between that and the ~5.5% short term treasury yield.

Marcus by Goldman Sachs claims their rate is "8x the national average" at 4.40% and I'm sure they're making money just fine too.