It would require compromised hardware all the way through the network to the listening device.
Discussion
Right. Which is why it's unlikely. But interesting channel.
I sometimes think about power analysis, picking up the high frequency signal through your home power lines.
One interesting point is that there are hard limits on ghostly comm channels. To avoid detection they have remain within plausible noise levels. This means that only a tiny fraction of total network information can be exfilterated without detection, which supports your point, that it is, mostly, not happening.