I have layman knowledge of this and do not understand. How does this compare to the level of security described in the GP comment (no connection at all)?
It might help to imagine there's a data diode between the interface box and the rest of the system. In an ideal world, there should be few differences beyond the actual data being sent. In practice, the analysis is a lot more complicated. I've also seen cases where there should be no connection hardware at all, but someone forgot to disable Bluetooth on a dev board or something and it shows up in a red team exercise.