> You know exactly what your program is doing

I think thd more accurate sentence would be "know exactly what the computer is doing". Lots of languages have clear and simple semantics that let you understand what your program will do. This is not the case for C though. The only way to understand what a C program does is to have a very complete mental model of the low level computer architecture, memory layouts, call stack organization and so on (aka knowing what the computer does). That is actually lot to go through for a first introductory course (hence why they are making a restricted language to teach before jumping to full C)