Your example just prints a man page. It doesn't do anything useful besides this.

I'm asking how argparse can do two incompatible things: provide a command-line interface for an end user and generate a man page in a build script.

Do you mean that the programmer should create a separate script for generating a man page and copy a part of the code from the main program? This workflow should be documented, with examples. And it is not applicable for simple one-file scripts.