Actually, Mr Cook makes a valid point about vertical touchscreen interfaces. Back in the 80's when they first came out, you got a lot of them in, e.g., kiosks and whathaveyou. And while they work for short periods (e.g., look something up real quick), for any extended period of time over a few minutes the arms get very tired, very fast. They're not meant to be held out like that for any long time, so you wind up with arms sore and fatigued, and feel like, yes, gorilla arms when you're through.
Usually an ideal angle for touchscreens is at an angle, preferably one which has a sweet spot for viewing, and some sort of support for the touching arm.
(Fun fact: the 'gorilla arms' problem is considred one of the main examples of what happens when you don't take real-world user experience into account. It's why companies that do extensive end user testing put out products that work better, and companies that just follow along with the trappings fall by the wayside.)