Nik, perhaps I can answer this, as it is a very simple matter. Let me first quote part of Yang Jun's post on the 'roots' topic (which is under the barehand 103-form forum):
"As to questions regarding the relationship of Fu Zhongwen to the Yang family, Yang Jianhou's second son Zhaoyuan (Yang Chengfu's second older brother, who died when quite young) had a daughter named Yang Cong who married a man from Yong Nian named Zhao Shutang. Zhao Shutang had a son named Zhao Bin and a daughter named Zhao Guizhen. Zhao Guizhen later married a man from Yong Nian named Fu Zhongwen. When Fu Zhongwen went to Shanghai to work in a textile factory, because he was a relative, he lived for a short time in the household of my great grandfather (Yang Chengfu). During the period that Yang Chengfu was in Shanghai, Fu Zhongwen learned Tai Chi along with others who were disciples of Yang Chengfu. Fu Zhongwen, because he was a relative by marriage, called Yang Chengfu 'San Lao Ye' or 'third maternal grandfather' and always referred to my grandfather and his brothers as 'jiu jiu' or ' maternal uncle'. Fu Zhongwen's relationship to the Yang family was that of a relative rather than a disciple. Later in Shanghai Fu Zhongwen practiced diligently and worked in a very positive way to promote and spread traditional Yang style Tai Chi Chuan. He had students all over China as well as all round the world. Fu Zhongwen was a grandmaster who made huge contributions to the development of traditional Yang Style Tai Chi Chuan."
As you can see from the quote, Fu Zhongwen was a relative by marriage to the Yang family, studied with Yang Zhenduo's father (Yang Chengfu) and was an important player in the same lineage. The site you have pointed out is run by Fu Shengyuan, Fu Zhongwen's son. Therefore that site represents another branch of the same lineage as Yang Zhenduo and Yang Jun. If you look carefully in the family tree page which you cited, you can find Yang Zhenduo.