It's not secret, it's updateable, and doesn't need to be running constantly, and doesn't need to be proprietary.
It's possible to just not load the modem firmware and have Pinephone as a device with two ARM SoCs both running mainline Linux and open bootloader. ;)
You'll just not be able to use EG25-G as a modem with modem FW not loaded. There are also options in between.