Do we need to understand the nature of Second Language (L2) writing in order to teach either the L2 writing or language?
It is not necessary to understand the writing system of the target language before it is taught. Instead, L2 teaching is an integrative process that has moved from BICS (Basic Interpersonal Communication Skills) to the CALP and CALLA (Cognitive Academic Language Proficiency/Approach).
The CALLA method integrates vocabulary, literature, interpersonal communication, specific-purpose language, and written communication. This being said, the print-rich opportunities found in the Internet, Social Media, books, magazines, blogs, and entertainment (lyrics from songs, hip-hop) etc., the understanding of the written language occurs in tandem with the understanding of spoken language when the two are brought together. The processes of permutation and concatenation (putting a string of words in a specific order to understand their intended meaning) occur naturally when the written language, either in L1 or L2, is presented.
When the language systems are different (i.e. Japanese, Chinese, etc), even the understanding of language is made simpler by connecting the sound to a symbol. even when the writing system is completely unknown it is no different than learning to read for the first time: recognize the symbol, connect it to a sound, build schema, and then reproduce the symbol.
Writing is a complex process that requires the author to be aware of and combine various components of language successfully. (University of Texas)
The writing process in a first language (L2) becomes automatic by adulthood but, when writing in a Second Language (L2), that writing process becomes "self-conscious" again (i.e., difficult to execute without deeply focused attention). It is because of this self-consciousness that extra tie for "idea generation" and writing planning are needed for L2 writing. Thus it is true that a teacher of L2 writing needs to understand the L2 before being able to effectively teach L2 writing.