Post doesn't make sense. It clearly states that you only be sure that package is working by actually installing it into clean virtualenv and testing it there. `src` or any other layout doesn't matter.
The difference is if you're running tests from your root project directory, the package is importable regardless of whether or not it is installed, as python picks up packages in the current working directory by name. src/ prevents this.
https://blog.ionelmc.ro/2014/05/25/python-packaging/#the-str...
It describes some of the outdated motivations for the other layouts commonly seen with python, as well as the many benefits of the “src” layout.