The firmware is not open source, that's entirely untrue. There is nothing open source about the firmware on peripherals or the firmware on the SoC. The only thing that's open source is the late boot chain and OS. It would be nice if people would stop spreading dishonest claims.
Not only that, but they've gone out of their way to turn the proprietary firmware into a black box that's much harder to inspect than other devices and cannot receive crucial security updates. They've taken the opposite path of opening it up, but rather have made it more closed.
The reason for that is their attempt at gaming / manipulating the FSF RYF standard, since it considers proprietary firmware fine as long as it's not possible to update it, which is incredibly stupid. It's crucial to provide full security updates and they've prevented doing that.
The hardware will ship with a bunch of serious, known vulnerabilities on day one with no way to fix them since they've gone out of the way to prevent doing it. They've also gone of their way not to meet basic security standards like offering verified boot on the main SoC too.
Can you provide links / sources to the outrageous claim that a proprietary ARM SoC and a bunch of other proprietary hardware does not have proprietary firmware? It's not me making the incredible and clearly false claims. The burden of proof isn't on me to disprove clear nonsense.