The tech companies aren't even underwriting the accounts. They are just resellers of accounts from actual banks, throwing on a fancy web interface and then data mining your transactions.
Is it common for boutique brokerages to offer banking?
They have over 6 million accounts, which is more than E*TRADE and within a factor of 2 of Charles Schwab or TD Ameritrade. And these brokerages apparently felt enough of a competitive threat that they all adopted Robinhood's commission-free model.
Regardless, even if you don't consider Robinhood to be a major brokerage, they clearly aspire to be one, so offering banking services makes perfect sense.
What's a company worth with a $1b in it that doesn't have to payout any interest on it and people are in no rush to get back (on average) ?