Air gap is a useful strategy. But what is that system? You don’t really know anything about its origin or what any of its processors actually do. You know really nothing about any of the firmware or software you run on it. Just getting software on to it securely is a huge challenge to prove its origin and the whole supply chain. And then getting data out is a whole other problem. A general purpose computer is not a great choice if you want the best in security. And having it just in your house isn’t that secure. Obviously as I say, most people don’t need the best security.