You don't have to serialize data or translate data representations between CPython and C. That article is wrong. What's slow in their example is storing data (such as integers) the way CPython likes to store it, not translating that form to a form easily manipulated in C, such as a native integer in a register. That's just a single MOV instruction, once you get past all the type checking and reference counting.
You can avoid that problem to some extent by implementing your own data container as part of your C extension (the article's solution #1); frobbing that from a Python loop can still be significantly faster than allocating and deallocating boxed integers all the time, with dynamic dispatch and reference counting. But, yes, to really get reasonable performance you want to not be running bytecodes in the Python interpreter loop at all (the article's solution #2).
But that's not because of serialization or other kinds of data format translation.
You can avoid that problem to some extent by implementing your own data container as part of your C extension (the article's solution #1); frobbing that from a Python loop can still be significantly faster than allocating and deallocating boxed integers all the time, with dynamic dispatch and reference counting. But, yes, to really get reasonable performance you want to not be running bytecodes in the Python interpreter loop at all (the article's solution #2).
But that's not because of serialization or other kinds of data format translation.