It seems that the band plays with backup music as they sound so full and big, but no backup is used in this case. Often times digital accordion players are "accused" of using backup as they can play multiple instruments/parts (bass, chords, treble, and percussion sounds)from a single digital instrument. Well, I heard some say that the accordion is the very first arranger as every chord button plays three notes instead of one....

It’s rather unusual indeed. Sounds like they had rehearsed it for thousand times. Nice crossover.

Also, a digital drum kit is used here. Perhaps an acoustic drum kit is hard to manage, which is transportation, amplification (or the opposite of it). Although, this digital kit doesn’t not sound exactly like an acoustic one.

Very interesting...My wife played accordion a long time ago...
To me it must be very hard to play...
I think they played it all with out PC driven backing tracks...
I really liked it...

Yes, I noticed the drummer was playing an electronic drum set. Always pros and cons. Also, I wouldn't be surprised if the accordionist assigned some percussion sounds to the bass or chord buttons to be played manually. Quite effective.