In the sense that the author uses the word "retarded" to mean that it takes Japanese longer to learn their language than other cultures then yes, the author is correct. Japanese students take until 6th grade to learn the 2,000 most common kanji characters. Most middle schoolers cannot read a newspaper. Heck even most high schoolers shy away from newspapers in Japan.

Manga (comics) are the way most Japanese learn their language. One of the reasons that manga are so helpful is their usage of furigana which are small hiragana (phonetic alphabet) characters printed next to the more difficult kanji characters (pictograms).

Japanese people come in a wide range of IQs just like any other culture. In my experience they are not any more stupid nor any more intelligent than the average U.S. citizen. They do often forget kanji characters, just like you would. It is common for Japanese to ask someone to jog their memory on how a character is written. In this case, people often respond by drawing the character on their palm or saying something like, "anshin no an" meaning it is the same "an" character as in the word "anshin". Language is very different when there are literally tens of thousands of characters to learn.