I apologize but I don't believe these things to be literally true. You can clearly speak at least one natural language with native-speaker fluency, and that inherently involves committing a large amount of raw data to memory that may never be "fully understood" in a complete sense. More generally, memorizing stuff effectively that one does not clearly understand happens all the time in all sorts of education, and my view is that genuinely effective methods can indeed make this roughly as easy as memorizing the lexicon of one's native language.

(The success of spaced-repetition software is at least an existence proof that such methods are in fact possible, though the context is not exactly the same as classroom instruction.)

Of course whenever "sufficient understanding" happens to be feasible it should be included; I have never said otherwise. What "Progressive" education means when it pushes "open inquiry" and "students learning by themselves" is something radically different, that in practice amounts to letting the students 'sink or swim' with no effective support whatsoever.