I was going to add "and Faraday Cage" until I got to

For the current experiment, researchers argue that malware that managed to infect an air-gapped [offline] computer can transform and modulate locally stored files into audio signals and relay them to another nearby computer via connected speakers, headphones, earphones, or earbuds.

Which still require infecting offline computer somehow (and connected machine in close proximity as well).

Point taken though, nothing is perfect. And if everything else fails, there's always social engineering :)