When I hooked a Raspberry Pi (model B maybe?) up to an analog input on my Marantz NR1605, the UI of the receiver got noticeably laggier until it was barely responsive. Disconnecting made the UI perk back up. I noticed the Pi had a 2.5V DC bias on its audio out. I didn't investigate further.

As for the article, I'm left with more questions. Surely the solar inverter wasn't running all the time in series, like a double conversion UPS, right? So how how would the mains waveform have been significantly affected in normal operation?

Also those scope traces, what's the scale? Are we talking 100mV dip, or a 1V dip? And is that a storage scope, or is that one cycle of the supply's ongoing ripple? And the complete lack of any dip on the "fixed" trace with extra capacitance makes me wonder if they even got the triggering right.