For folks trying to understand this, some context which may be useful:
Checking accounts are loss leaders virtually everywhere, the exception being smaller community banks. Their primary revenue stream was, once upon a time, net interest income, but these days due to the extremely low interest environment and alternate sources of funding the revenue stream is more weighted towards fees (primarily NSFs, although that was hit a few years ago) and debit card interchange.
Robinhood also likely expects to not become the park-your-money account of choice for older dentists but rather to become the spend-your-money account for their millennial userbase. With high velocity of money and low balances the interest expense is minimal and, to the extent they use debit cards, the interchange revenue can be material. (In a stylized example where someone makes $2k a month and spends $200 on debit card purchases and $1.8k on rent/etc the interest cost for the year is ~$30 and the debit card interchange for the year is ~$60, even ignoring potential interest revenue.)
This is roughly in the same line as their core strategy, which is spending what would otherwise be a marketing budget on keeping commissions at zero, making money on the other ways brokerages make money. If you do not understand how a brokerage makes money, I encourage you to peruse the annual reports of e.g. eTrade or TD Ameritrade, which will happily explain their revenue sources and why commissions are a surprisingly small portion.
Metacomment: geeks who believe they have outmathed a financial firm should ask themselves "Are financial firms likely to be bad at math?" and "Are financial firms incapable of hiring their own geeks?"