The title of this post is a question that seems to be perpetuated in various forums.The implication being that he didn't. My favourite one is that Ch'eng Man Ch'ing didn't appear in the "training diary" of Yang Chengfu, who was , I believe, illiterate. Work that one out!
However, in Louis Swaims translation of Yang Chengfu's " The Essence and Applications of Taijiquan" he has a passage on page xi which is a quote from Yang Zhenji, 2nd son of Yang Chengfu, which states "Taijiquan tiyong quanshu was written by my father's disciple, Zheng Manqing, according to my father's performance narratives and requirements. This is factual."
Good enough for me. No comments required.