I have just been on a Cathay Pacific flight and BadmintonWorld was one of their selections on the inflight entertainment.
What BadmintonWorld doesn't emphasise is Tang's contribution. It's not a coincidence China men were playing catchup to Indonesia all the way up to 2000 and then this reversed when Tang went to China.
I do not think China was playing catchup with Indonesia up to 2000. China was not inferior to Indonesia before 1980, we can maybe go by the results from the Asian Games. I clearly remember HK Invitational (1977?) where Yu Yaodong clearly outplayed Liem Swie King and during the 80's where we see players like Chen Tianlong, Luan Jian, Yao Ximing/Sun Zhian, Han Jian, Yang Yang, Zhao Jianhua, etc. In my opinion, China was not behind Indonesia in terms of badminton, maybe the other way around.
I do however think, Indonesia recovered their top position due to Tang Xinfu's contribution, by becoming their national team coach. After taking over the position (I believe never acknowledged officially by Indonesia) he produced the players like Kurniawan, Ardy, Kusuma, Suprianto, Susanto, etc which where the top players. China on the other hand was struggling due to the retirement of their top coaches (Wang Wenjiao, Hou Jiachang, Chen Fushou, etc) and changes to training methodologies and tactical approaches by new technical staff. Chinese players struggled at that time to stay on top of the rankings regularly.
China did regain the top spot after Tang Xinfu returned to China and entrusted the coaching position. He was able with his work, transfer his knowledge and experience to the players we still see today. I would say, to China, Tang Xinfu's contribution as a player was during the 60s and 70s'as a coach until mid 80s and then from 1996 onwards. But to the badminton community, I don't know if his training methods would be a legacy to follow.