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.