I think sometimes people get etiquette confused with "I don't like x therefore anyone doing it in my presence is rude". I see that on this thread. Just because one does not like violins, it doesn't make what they did rude. Other people were no doubt enjoying the performance. So many people seem to expect that the world be tailored to their preferences and that they should never have to come across anything that they don't like. Sometimes we have to give way to others preferences. That's just the way the world works and it has nothing to do with etiquette.
One must ask, though, why those of us who want a bit of (reasonable, sort of achievable) peace & quiet on the plane are the ones who must bend. I'm getting flashback to the thread where people were singing in the ICU waiting room.
I've been on a plane that was stuck on the tarmac for over an hour. There was no peace and quiet. Everyone was rustling in their seats, talking, getting on their cell phones and conversing loudly, complaining to the flight attendants, and the children on board were starting to scream. Luckily they deplaned us, but I can't imagine that staying stuck on the plane for longer would constitute anyone's definition of peace and quiet.
I have anxiety attacks, especially in crowds or places that I can't escape easily from. In an airplane, when I'm stuck and everyone else is getting antsy, I have been known to force myself into breathing exercises or start humming quietly to myself because sometimes music is literally THE ONLY thing keeping me from sobbing hysterically. I would have welcomed this music, or any music. It likely would have been a lifeline to me.
I understand other people have other conditions and may not have appreciated this, but I am the flip side. In 3 hours of playing music from my phone without charing it, it would have been dead. And without music to keep me calm, I would have been an absolute wreck. I don't like Dvorak, there are much better classical pieces/composers that I would prefer, but this music could literally have been my saving grace if I was in that situation.
Was it rude? Unless someone had raised objections and they kept going despite them, no. This group set up in the seats, there were other people scrambling around holding their music for them, the cello was angled into the aisle. I can't imagine how it could have come as a surprise to anyone that they were going to play. There was plenty of time to speak up if someone had a problem with it. It would appear that nobody did, so they played a relatively short piece and then went back to waiting. I don't see how that could be construed as bad.