The idea that plugging the device into a laptop or other data-capable device creates an attack surface that doesn’t exist if all its electricity comes from a battery

This is as air-gapped as it gets!

I read this in Tools of Titans, by Tim Ferris(Sammy Kamkar section)

USB is insecure, but the cold power uses a battery and even if it was compromised, the cable attaches to a magnet so there's no way to transmit data to it.

Even if this is not in your threat model, however, it makes it easier to sign transactions in a bank vault where there might not be an outlet.