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.
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Interesting - "Amazon calculates a product’s star ratings using a machine learned model instead of a raw data average. The machine learned model takes into account factors including: the age of a review, helpfulness votes by customers and whether the reviews are from verified purchases."