What's your definition of zoomable? I'm able to adjust text size just fine. More details on your specific issue? If you mean layout, it is responsive. The text column narrows and the images never exceed 100% width.
The author is stating that the child dependency cannot be extracted to another library or app. If it is extracted, it is untested, because the only tests wrapping the child dependency are actually testing the child's original parent. (Which is likely to not exist in whatever other library/app to which the child component is moved.) And then, to retroactively add tests to the child component in order to facilitate moving it to another library, is painful.
Having symmetrical tests enable components to be easier moved to other libraries/apps because the test can move with the unit under test.
There is a strong difference between exposing everything as part of the API, and making it permanently inaccessible. Taking your car example, imagine if the engine were in a locked black box. Any modifications or changes necessary would require replacing or duplicating the ENTIRE ENGINE. Instead, those with the proper know-how, can get under the hood when necessary. Making the engine accessible doesn't force drivers to know everything about it. It's hidden, but accessible when needed.
To round-trip this analogy, I can't edit the source code of a system library or purchased component unless I have the source, and have the rights to edit it, neither of which are a given in today's world.
Sure, an epoxy-encased engine isn't going to let me tinker with it, but I will never tinker with my engine. That's one of the wonderful things about separation of concerns that OO gives me, including all the perceived badness of enforced privacy: I can work on my particular thing without needing to know anything about the innards, because it is not my responsibility to do so, neither explicitly through delegation of concern and documentation, or implicitly through presentation of a semi-private API.
If I'm able to see it, then I'm expected to see it, and that blurs the lines of the API.
Depends on your definition of frontend tests. TDD alleviates these workflow problems for javascript (the actual code of the application). But using automated tests for the UI layout is gonna be a bad time. As @davemo mentioned, this post seems primarily directed at a designer's workflow with CSS and less about javascript development.
