Even if you have the full menu on your phone, you still have to zoom into the menu to read it and pan the view. I don't see how that's any different from scrolling through a categorized menu in a website on your phone.
The difference for me is when the menu gets large then it becomes laborious to scroll through a long linear menu vs a rectangular representation of that same data - which allows for much shorter scrolls. Imagine the menu on a restaurant was handed to you on a 4ft long receipt, you’d get pretty fed up of moving it up and down.
Zooming allows you to narrow in on the content you desire from a big picture point of view while vertically scrolling you just must hope that you get lucky to find what you are looking for.