SouthPark I also posted something around these lines but due to the problem I had to restore the forum and that post was lost and I was busy with restoring the files back.
Indeed, the USB-audio signal should not be affected by any interference since it's a digitally encoded signal.
However, now that I think more of it, it's possible that there's some analog to digital conversion happening on the piano (e.g. the audio input) which might get some interference and noise that gets digitized and sent through the USB-audio line. I believe @Koobenot has a Numa Compact 2x and not a Numa Piano and they may be different in that regard. On the Numa Piano the audio input has a noise gate, so it won't introduce audio noise below a certain threshold in the audio path (and then in the USB-audio line).