While your statement is technically correct, in practice C is a very different beast from C++/Obj-C. For example, in C dealing with global state is a important

While your statement is technically correct, in practice C is a very different beast from C++/Obj-C. For example, in C dealing with global state is a important problem, while one would just encapsulate the state in a object in C++. So someone who learns just one of the supersets can easily miss all the details which the additional features of C++/Obj-C hide.