That's mostly irrelevant, since that only protects the integrity and confidentiality of that TCP connection. However, you can still exploit the vulnerabilities

That's mostly irrelevant, since that only protects the integrity and confidentiality of that TCP connection. However, you can still exploit the vulnerabilities of all the services listening on the device. Separating voting machine networks from the internet is orthogonal to the problem of network protocol hardening between endpoints within this "separated" network.