They're talking about devices known to be extraordinarily insecure, which are still closed source hardware with closed source firmware. Having schematics for the board does not avoid trusting the hardware. It's still a closed source SoC and the same for the other components such as the SSD, Wi-Fi, Bluetooth, cellular, etc. but those components are much less secure without proper updates and security protections. The whole point of an SoC is that it has the complexity of a traditional CPU, GPU, motherboard and other components merged into a single chip, and that's entirely closed source with closed source firmware on those devices.