The boss "host" from Splintered States is just a random server box, considering it serves no purpose save for protecting one piece of paydata which it does pretty poorly (because it has no way to communicate with anything or anyone else that there's a breach). That's the thing about stuff on the matrix. If you say it can't be reached, they it can't reach the rest of the matrix to do anything. It's about as useful as a datachip (maybe less so). It'd probably been better protected as a datachip in a safe that was slaved to the actual security host. Or just one left haphazardly in a dusty old room. That'd be way harder to find at least.
If the entire matrix access on Governor's Island is restricted to that area, that's not the host being inaccessible from the matrix, that's the entire grid being blocked off from the rest of the island. Which means that anyone on the island who's not in that compound may not be able to use the matrix at all. I haven't read that part of the book yet though, so many how I understand that sentence doesn't make sense.
