Only with multiple caveats.

The first line on the page is "Mercury is a fintech company, not an FDIC-insured bank.".

Ultimately the money is stored in actual banks. We're stuck with "fintechs" because banks seemingly can't/won't modernize and the government won't allow new banks to be created.

At least I know that bunq, an European „NeoBank“, applied for an FDIC license in the US and received one. It’s offers a lot of cool features and an API.