No, it’s not just the internet/BT/any other sort of connectivity that makes an Electronic system vulnerable. As the names implies Electronic (electrical/digital Input, Process, Output) even an isolated machine with punching buttons can be progrmmed that certain button press has a glitch or programmed to manipulate. Here we talk of a system where even 1% error might be unacceptable and no physical trail. There is electronic system or machine that can be referred to as Perfect.
It’s not always the Hacker it’s Phisers and Manipulators who can manipulate the data with leaving no physical trace. The system that can deploy Survellience Cameras in Senate Election Booths can go to any length!!