"Considered harmful" is the code word for passive-aggressively manipulating an opinion as truth. Whenever I see it, I would mentally say, fuck you, just state your opinion as opinion and don't try to twist it around.

Back to the blog. Contrast to the claim, closure for class/module privacy is great because it separates inner implementation from public interface. I really don't want any outside code to touch the private code. Because tomorrow I might change the inner implementation, function signature, break the function into two pieces, or proxy-call other libraries. And I want the assurance that the change won't break any calling codes.

Public function is like global variable. Anyone can access it anywhere and anytime. You don't know how, where, and when people call it. If you change its signature, you pretty much have to hunt down every call of it to retrofit. Private codes have no such issue. It's best to limit the scope of codes that are intended to be private.

Testing private code is a non-issue. Nobody outside cares about your private functions. They care about your public functions serving as the public interface. You test your public functions as your users would call them. Your public functions in turn exercise the private functions.

If you really want to test the private functions, write private tests in the same scope as the private functions to call them.