Technically, this isn't completely true 

There are quite a few paper about how to exfiltrate data from air-gapped computers.
Those techniques are highly sophisticated and the chances of happening to are close to zero. But some would include:

• AirHopper: Malware to encode data into FM signals transmitted from a screen cable. This signal can be received by any smartphone with an FM receiver
• PowerHammer: Exfiltration via Powerline: With probes on the computer and the power control box, malware on the air-gapped computer can increase/decrease the cpu load by doing useless (but ressource heavy) calculations to transmit data via the power line.
• Another option requires a camers to be installed close to the computer: Using the hard disk led's to transmit data.

Those are not just theories, but they have been proven to work.
There are a few more extremely fascinating (and highly unlikely) attacks which could extract data from such an air-gapped setup.
Quite a few paper have been published which cover exactly that: Exfiltrating data from air-gapped computers. They are quite exciting to read.

It is obvious that no typical crypto holder will face such an attack, altough its interesting to know which techniques exist