That is a big part of the author's point: the library ecosystem is here for Python today. While there is a heavy penalty for anything written in Python itself,

That is a big part of the author's point: the library ecosystem is here for Python today. While there is a heavy penalty for anything written in Python itself, it doesn't really matter since there isn't much of a penalty once the data is passed to highly optimized libraries and those libraries allow developers to select efficient algorithms rather than implementing their own algorithms (which are likely to be less efficient).