Unless the article was updated after your comment: the reason is right there in the article:

"However, Python does not produce any instructions for assert statements when compiling source code into optimized byte code (e.g. python -O). That silently removes whatever protection against malformed data that the programmer wired into their code leaving the application open to attacks.

The root cause of this weakness is that the assert mechanism is designed purely for testing purposes, as is done in C++. Programmers must use other means for ensuring data consistency."