Note carefully that he is not saying that you should always practice in the way you would in performance. This harkens back to Jeff Berlin's pedagogy of separating academics (learning) from art (performance). That is to say, when you are spending hours just learning stuff (picking out notes for repertoire, transcribing, figuring out best fingerings, analyzing chord progressions, etc.) then you want to be as comfortable as possible and may have to shift from standing to sitting and vice versa, adjusting hand position, letting bass rest on legs, etc.
Then, when you know your performance repertoire cold, then you should go to rehearsal mode where you model everything exactly as you want it to look and sound on stage.