I am not saying it is, normally old systems are the least secure. The bit you’re not getting is that this system is almost certainly air gapped, just not by choice. It can’t work with modern networks. It can’t work with modern viruses. Any exploit a modern hacker would think to use probably doesn’t exist yet. It’s a bit like trying to break someone’s car by putting sugar in the fuel, except they ride a horse. Do you get it yet?