What's open about the Hologram Nova is the part they made: the board adapting the used cellular modem chip to USB, and a software SDK around it. All the cellular functions are in that chip, whose design and firmware aren't open.
Open implementations of cellular tech and protocol stacks exist, so yes, it's possible to do it. Making a consumer product from them would be a massive extra step though, and might very well be not feasible in practice.