1. Because a big part -- perhaps the biggest part -- of "learning to program" is understanding how to move between the picture in your head and the text that makes it real. From both directions: "What does this code do?" and "What code do I need to write to make this happen?"
Purely visual programming are, IMO, noble lies.* You want to start with an environment that echoes the final state, not one that's totally dissimilar.
e.g., training wheels vs. computer bicycle simulator.
*: The exception to this is when you're teaching children ~13 or younger, in my experience. It's always a question of what mental models a student has at their disposal.
2. By the time a student is at this stage, they have more sophisticated, general models the teacher can rely on. In this essay Brett is working in a world where someone might not have the right mental model for assignment, variables, looping, etc.