Even Beethoven would ask it, anyway in 2016 one cannot go around wearing a sack. It would be out of place... it's as if I wore a big white wig to play. Come on.
Agreed: you don't go around, and even less so on stage, wearing a sack.
But not even in a way that the focus is blatantly on you (the performer) and not on what you are performing, perhaps even egregiously so.
Finally I saw the video. Well... to tell the truth I didn't see it, I just limited myself to listening. Anyway... excellent performance! I agree with whoever said "From a musician I only ask to play" - I add "well". The rest is secondary to me, although I still always expect appropriate attire.
P.S.: Today I finally met the great Armando in person! We also talked about this, among other things.