12tone: Is Brahms' composition impossible to play?

"Sometimes, there are musical ideas so complicated that they wind up being not just physically impossible, but conceptually impossible as well. But did Brahms write one of those? Well... It's complicated" - an interesting new music theory video by 12tone about Johannes Brahms' composition Variations on an Original Theme, opus 21, number1, which is supposedly just impossible to play. Watch the video below.