That makes sense. On the other hand, what made me side with the author here is this.
About a year ago, I read a study (which I wish to hell I could locate again) that made the case that children raised with more phonetic languages, such as Spanish and Finnish, had a notable advantage over children whose native language is English, because they learned to read and write much earlier. While American, British, and Australian children puzzle over words like “plough,” “epitome,” and “Worcestershire,” children in Spain are steadily progressing through more and higher-level books, enabling them earlier access to advanced skills such as reasoning, synthesis, and discussion. They simply read at a higher level than English speakers of the same age.
Spanish speakers are expected to read arbitrary words and write (albeit with lots of spelling mistakes) by (optimistically) kindergarten or early first grade. The written-spoken language is very regular so it's very easy so "sound out" words. This is in contrast to what I hear is like to teach English speaking children. So yes, the author's hypothesis seems plausible to me.