The easiest counterpoint to this is to think about a HTTP request. If your library is called by one route, and is known to fail in certain circumstances, this failure should not bring down the entire system. A well designed library should generally not make the choice to crash a system - that's the caller's decision. Yes there are exceptions to this, which is why TFA stated this as "rare" not "never".

Using the assert macro in your code is (in my experience) generally bad. If your code is written well, you can never test that code path. Document invariants with tests instead, or better yet with infallible code.