This is true by definition, at least by the definition of a "writing system" commonly used by DeFrancis and others. For the Chinese system, it is true that the vast majority of written characters are based on sound, but there are also characters that are primarily semantic in construction (e.g. pictographic, at least in origins).
The more complicated (and somewhat philosophical) question is whether or not it make sense to call something a language (or writing system) that is not based on speech, or cannot be represented as speech. Symbolic logic, for example, includes a formally defined set of symbols that represent meaning in a consistent and intelligible way. How we discuss these sets of symbols is a very interesting topic (albeit rather distinct from the topic at hand).