Using setup.py does not mean "not using extra tools". It depends on setuptools, which is an "extra tools" just like flit (used in the article) or any other tool. In fact, with using only setuptools one will need a whole additional set of tools to manage things like:

    * virtual environments (granted, venv is now part of the stdlib, but it's still an "extra tool")
    * publishing to PyPI or another index (twine)
    * dependency management (both for development and actual dependencies)

Plus the tools that are needed anyway, to manage common development actions like:

    * unit tests (pytests)
    * static typing (mypy)
    * linting (flake8, pylint)
    * styling (black)

The article is correct in using pyproject.toml, which has become the standard way to specify build mechanism for your package [0]. Even setuptools supports it in the latest versions [1], meaning that setup.py is becoming obsolete, or at least unnecessary.

Finally, tools like Poetry [2] offer a whole set of functionalities in one place (dependency management, virtual environments, publishing), which means that they need fewer "extra tools" than just setuptools.

[0] https://www.python.org/dev/peps/pep-0518/

[1] https://setuptools.readthedocs.io/en/latest/build_meta.html

[2] https://python-poetry.org/