I agree that cybersecurity features should be included. In fact I think they should be included for free. The problem is that Microsoft wanted to charge the Department of Defense and it sounds like they used politics to make sure they could, and if true then they (and maybe also the DoD?) may have violated some federal laws around government procurement and “gifts” from contractors to the government.
LineageOS Android TV looks like an interesting solution but having SELinux in permissive mode is a bit scary. Maybe someone with more in-depth Linux knowledge can chime in here but my understanding is that would mean that the Android kernel’s security system would basically be turned off. Permissive Mode logs security access violations within the OS but does nothing to prevent them so I would be wary of connecting it to the internet.