The point is having the digital transmission simple enough that you can use fully *physical* filters to discard everything but the 3-4 messages you need through. Of course it is still digital, but my point is, it is a physically enforced studied, safe subset and doesn't allow for full network access. There are of course a variety of ways to further firewall that, but something needs to act like as much of an air gap as possible while allowing for remote communications.