Mondo CTO here. Good question, thanks. It's "full stack" in the banking industry sense, not the "full stack web developer" sense.

Most challenger banks don't pursue a banking license and have to work with an existing licensed bank. This means they are constrained by their processes and legacy technology (think cobol on mainframes). We don't have those problems because we own the full "stack", from the user interface and the connectivity to the payment schemes all the way down to holding the actual funds with the central bank.