Further in the article he discusses the 'virtuous cycle'; connections between writing, speaking, and reading. With phonetic alphabets, the way something is spoken reinforces the way that it is read, the way it is read reinforces the way that it is written, the way that it is written reinforces the way that it is spoken (for the most part, even English fails this cycle sometimes). However, with character sets the cycle is broken, and the speaker has to learn 3 different memorization techniques.
Regarding your question, there is a difference between not knowing how to write a word and not knowing how to spell a word. If someone in English doesn't know how to spell 'sneeze' or any other word, they can at least come close enough and convey information 'fuzzily' via text using an incorrect spelling. Now that I'm writing this, though, I suppose with character sets like Chinese if you know characters that are close enough you potentially could use other characters to convey the information, like mouth-fart for sneeze or something along these lines. But I don't speak the language, so that is just a theory.
Do Chinese speakers use the language this way if there is a character they don't know how to write?