How did that conclusion, from “no books” only? The point is not literacy per se or something. It's more about how much you use a language in your daily life. If

How did that conclusion, from “no books” only? The point is not literacy per se or something. It's more about how much you use a language in your daily life. If it's only limited to talking to just your parents on occasional basis then you miss out on a lot of language skill development. If you use Cantonese for 1 hour in a given week total and the remaining 110 hours of your waking time are filled with English, it's not much of a surprise what language ability gets developed more. Cantonese is still your first language – you learned it first, in limited capacity – but it can cease being your primary language.