And I was just ... bewildered. It would never have occurred to me, growing up, that not knowing the words in a book might be considered a reason to stop reading the book. If the subject interested me, and I had to trot to the dictionary a second or third time, I simply lugged the dictionary back and plunked it next to the book, and alternated between the two. I have some early memories of doing that, but probably not past 8 or so. I was reading at the adult level at least by the time I was 6, possibly earlier. My vocabulary got so big, so fast, that it quickly became rare for me to find new words unless I went entirely outside my knowledge sphere. That was actually part of the reason for me bookworming my way through a substantial portion of the Danville public library in junior high and early high school, new-word-hunger. (I can understand that it wouldn't be prudent to force someone to read a book that they couldn't understand, but frankly, I never saw concern about that; if a book was assigned, you had better handle it, and if you couldn't, tough. The only books anyone ever tried to take away from me were ones I'd picked out myself.) The books that fascinate me the most are the ones that take me into new territory.
Neither would it have occurred to me to abandon a book just because it was hard to understand. If the topic was interesting, I would reread challenging sections, or look for other references and then come back to see if the hard one made more sense, or ask my parents for an explanation, or find some adult who knew about that topic and pester them (not excepting total strangers, at times).
I can even remember that kind of persistence with a few fiction books, that I just couldn't get into but for some reason seemed like they ought to be interesting. Some became interesting later; others never did. But really, there are only two things that have a high likelihood of bouncing me out of a book: it's badly written and/or it bores me. It's possible for a book to be so far over my head that I have no interest in it, but the percentage of recognized words and concepts has to be minute and far from anything I might find useful. I have puzzled my way through a page or few of writing in languages I'm not even fluent in just for the fun of hunting for English borrowings or words that are close enough to some other language or root-word for me to recognize them.
This illuminates for me some of the reasons why I'm so different from most people, if those leveling techniques are at all common as they are described to be. There is my innate fascination with words, which causes unfamiliar words to be attractive rather than off-putting. There is the looping effect of seeking books to explain things I've encountered elsewhere, and seeking people to explain books. There is the context that my parents let me read whatever I wanted, whether it was at an "appropriate" level or not; and my disgust and outrage at other adults who occasionally tried to part me from books they considered inappropriate. I think anyone with an indelible attraction to words will tend to develop a larger vocabulary, even in the absence of outside encouragement; that anyone in a supportive environment will tend to develop a larger vocabulary than they would on their own, even if they aren't especially interested in words; and that combining the two probably accounts for many of the people with the largest vocabularies. There's logic to that, when you look for it.
But still, it seems utterly alien to me that not knowing the words in a book, or not immediately understanding its content, would be reasons to put it down.