I doubt they'd be able to 100% buy tbills at that maturity length (hence curiosity on what they're doing on the backend here), but even assuming 2.30~ average plus whatever Mastercard kickback is negative 50-70bp is still a hell of a margin to make on selling consumers tbills and not raising funds for your own investment activity

edit: a better way to explain it - we'll give you tbills at 3% p.a and let you pull out/in whenever you want and handle the rest is a hell of a business model without the rest of the story

My guess is: they're investing in 6-month T-Bills, they're expecting rates to rise over the next 6 months to 1 year, they're covering the remainder out of pocket, and by doing this they're capturing the lion's share of their target market before their competitors do.

In addition, I'm guessing their AUM are low relative to their popularity. Lots of millennials may be very vocal about Robinhood, but may not have the quietly massive asset balance that boomers have, tucked away in Schwab, Fidelity, or Vanguard. This could be a land grab for AUM to support future VC rounds, an acquisition, or an IPO.