Some repetition is good and children in fact often repeatedly read their favorite book or watch their favorite show. However, a key thing to keep in mind with Paul Nation's study is that "repetitions" of a word represented encountering it in different contexts.
This is important for three reasons. First is polysemy, which he mentioned. This is words with multiple meanings like fair (reasonable/just / light or unblemished / a type of public event with entertainment and vendors).
The second and in my opinion even larger issue is gaining and understanding of how the words are used and their scope. E.g., there's no word in Chinese that quite matches "nose" in English. The closest is 鼻子, but that can refer to an elephant trunk or a pig's snout, which "nose" can't. In some languages, there's a word that can be used for humans and pigs, but not elephants. In others, the same word also encompasses bird's beaks. The only way you'll learn this is from encountering the word in a lot of different contexts, not from drilling it repeatedly in the same context. Furthermore, there are a lot of words that tend to be used with or near each other but near others (collocations). In English, it's normal to say you're doing "pretty good" or "absolutely fantastic", but saying you're doing "absolutely good" would be very strange.
Finally, there are a lot of shared cultural stories each language community has. In English, these would be from Christianity, from classical Greco-Roman figures like Aesop, from German storytellers like the Brothers Grimm, other European storytellers like Hans Christian Anderson, etc. In Chinese (or Korean, Japanese, Vietnamese or other nearby countries) people are going to have a lot of shared stories from ancient China—philosophers from the Spring and Autumn era, historical stories and dramatizations of the interesting times in history such as the Three Kingdoms era, and many, many folk tales. If you don't understand at least the core of the cultural cannon, you'll regularly be confused by things in TV shows like soap operas or variety shows, even if you understand every word in transcript. Reading fiction will be even harder.
The more distant the language is from the one you speak natively, the more the second and especially third points will impact you unless you take in an enormous amount of input.